SemiHosting - TypeError: string argument without an encoding
I try to get debug messages via semihosting from a nrf52840 device and each message I get TypeError:

```3631888:ERROR:gdbserver:Unhandled exception in handle_message: string argument without an encoding
Traceback (most recent call last):
  File "/home/joerg/.local/lib/python3.8/site-packages/pyocd/gdbserver/gdbserver.py", line 418, in handle_message
    reply = handler(msg[msgStart:])
  File "/home/joerg/.local/lib/python3.8/site-packages/pyocd/gdbserver/gdbserver.py", line 666, in v_command
    return self.v_cont(cmd)
  File "/home/joerg/.local/lib/python3.8/site-packages/pyocd/gdbserver/gdbserver.py", line 717, in v_cont
    return self.resume(None)
  File "/home/joerg/.local/lib/python3.8/site-packages/pyocd/gdbserver/gdbserver.py", line 603, in resume
    was_semihost = self.semihost.check_and_handle_semihost_request()
  File "/home/joerg/.local/lib/python3.8/site-packages/pyocd/debug/semihost.py", line 441, in check_and_handle_semihost_request
    result = handler(args)
  File "/home/joerg/.local/lib/python3.8/site-packages/pyocd/debug/semihost.py", line 519, in handle_sys_write0
    msg = self._get_string(args)
  File "/home/joerg/.local/lib/python3.8/site-packages/pyocd/debug/semihost.py", line 478, in _get_string
    target_str = six.ensure_str(bytes(bytearray('')), encoding="ascii", errors="ignore")
TypeError: string argument without an encoding
```
BTW. I use a Pitaya-Link, and I don't know if this issue is related to pyOCD or Pitaya-Link firmware.
diff --git a/pyocd/debug/semihost.py b/pyocd/debug/semihost.py
index e5c91b1f..616500ef 100644
--- a/pyocd/debug/semihost.py
+++ b/pyocd/debug/semihost.py
@@ -1,5 +1,5 @@
 # pyOCD debugger
-# Copyright (c) 2015-2019 Arm Limited
+# Copyright (c) 2015-2020 Arm Limited
 # SPDX-License-Identifier: Apache-2.0
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -21,8 +21,10 @@ import logging
 import time
 import datetime
 import six
-import pyocd
+
+from ..coresight.cortex_m import CortexM
 from ..core import (exceptions, session)
+from ..utility.compatibility import byte_list_to_bytes
 
 LOG = logging.getLogger(__name__)
 
@@ -215,8 +217,10 @@ class InternalSemihostIOHandler(SemihostIOHandler):
         data = self.agent._get_string(ptr, length)
         try:
             f = self.open_files[fd]
-            if 'b' not in f.mode:
-                data = six.text_type(data)
+            if 'b' in f.mode:
+                data = six.ensure_binary(data)
+            else:
+                data = six.ensure_str(data)
             f.write(data)
             f.flush()
             return 0
@@ -409,8 +413,7 @@ class SemihostAgent(object):
           debugging breakpoint.
         """
         # Nothing to do if this is not a bkpt.
-        if (self.context.read32(pyocd.coresight.cortex_m.CortexM.DFSR) &
-                pyocd.coresight.cortex_m.CortexM.DFSR_BKPT) == 0:
+        if (self.context.read32(CortexM.DFSR) & CortexM.DFSR_BKPT) == 0:
             return False
 
         pc = self.context.read_core_register('pc')
@@ -473,9 +476,9 @@ class SemihostAgent(object):
     def _get_string(self, ptr, length=None):
         if length is not None:
             data = self.context.read_memory_block8(ptr, length)
-            return six.ensure_str(bytes(bytearray(data)), encoding="ascii", errors="ignore")
+            return six.ensure_str(byte_list_to_bytes(data))
 
-        target_str = six.ensure_str(bytes(bytearray('')), encoding="ascii", errors="ignore")
+        target_str = ''
         # TODO - use memory map to make sure we don't try to read off the end of memory
         # Limit string size in case it isn't terminated.
         while len(target_str) < MAX_STRING_LENGTH:
@@ -486,14 +489,14 @@ class SemihostAgent(object):
 
                 # Found a null terminator, append data up to but not including the null
                 # and then exit the loop.
-                target_str += six.ensure_str(bytes(bytearray(data[:terminator])), encoding="ascii", errors="ignore")
+                target_str += six.ensure_str(byte_list_to_bytes(data[:terminator]))
                 break
             except exceptions.TransferError:
                 # Failed to read some or all of the string.
                 break
             except ValueError:
                 # No null terminator was found. Append all of data.
-                target_str += six.ensure_str(bytes(bytearray(data)), encoding="ascii", errors="ignore")
+                target_str += six.ensure_str(byte_list_to_bytes(data))
                 ptr += 32
         return target_str
 
diff --git a/pyocd/utility/compatibility.py b/pyocd/utility/compatibility.py
index 32822f42..1b41a085 100644
--- a/pyocd/utility/compatibility.py
+++ b/pyocd/utility/compatibility.py
@@ -16,6 +16,7 @@
 
 import sys
 import functools
+import six
 
 PY3 = sys.version_info[0] == 3
 
@@ -60,6 +61,17 @@ else:
         else:
             return v
 
+if PY3:
+    def byte_list_to_bytes(data):
+        """! @brief Convert a sequence of integers (bytes) to a bytes object."""
+        # This bizarre construction
+        return bytes(data)
+else:
+    def byte_list_to_bytes(data):
+        """! @brief Convert a sequence of integers (bytes) to a bytes object."""
+        # This bizarre construction
+        return bytes(bytearray(data))
+
 # Make FileNotFoundError available to Python 2.x.
 try:
     FileNotFoundError = FileNotFoundError
diff --git a/test/unit/conftest.py b/test/unit/conftest.py
index e884fe04..b62bbb8f 100644
--- a/test/unit/conftest.py
+++ b/test/unit/conftest.py
@@ -33,7 +33,7 @@ def mockcore():
 def mockcore_no_fpu():
     return MockCore(has_fpu=False)
 
-# Ignore semihosting test that currently crashes on Travis
+# Ignore any tests that are currently causing issues.
 collect_ignore = [
-    "test_semihosting.py",
+    # None right now.
     ]
diff --git a/test/unit/test_semihosting.py b/test/unit/test_semihosting.py
index 310fb891..af7a8f68 100644
--- a/test/unit/test_semihosting.py
+++ b/test/unit/test_semihosting.py
@@ -1,5 +1,5 @@
 # pyOCD debugger
-# Copyright (c) 2006-2019 Arm Limited
+# Copyright (c) 2006-2020 Arm Limited
 # SPDX-License-Identifier: Apache-2.0
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -20,11 +20,13 @@ import sys
 import logging
 from elapsedtimer import ElapsedTimer
 import telnetlib
+import six
 
 from pyocd.core.exceptions import TimeoutError
 from pyocd.core.helpers import ConnectHelper
 from pyocd.core.target import Target
 from pyocd.debug import semihost
+from pyocd.utility.compatibility import byte_list_to_bytes
 
 @pytest.fixture(scope='module')
 def tgt(request):
@@ -126,7 +128,7 @@ class RecordingSemihostIOHandler(semihost.SemihostIOHandler):
             return length
         d = self._in_data[fd][:length]
         self._in_data[fd] = self._in_data[fd][length:]
-        self.agent.context.write_memory_block8(ptr, bytearray(d))
+        self.agent.context.write_memory_block8(ptr, bytearray(six.ensure_binary(d)))
         return length - len(d)
 
     def readc(self):
@@ -168,7 +170,7 @@ class SemihostRequestBuilder:
         argsptr = self.setup_semihost_request(semihost.TARGET_SYS_OPEN)
 
         # Write filename
-        filename = bytearray(filename + '\\x00')
+        filename = bytearray(six.ensure_binary(filename) + b'\\x00')
         self.ctx.write_memory_block8(argsptr + 12, filename)
 
         self.ctx.write32(argsptr, argsptr + 12) # null terminated filename
@@ -195,7 +197,7 @@ class SemihostRequestBuilder:
         argsptr = self.setup_semihost_request(semihost.TARGET_SYS_WRITE)
 
         # Write data
-        self.ctx.write_memory_block8(argsptr + 12, bytearray(data))
+        self.ctx.write_memory_block8(argsptr + 12, bytearray(six.ensure_binary(data)))
 
         self.ctx.write32(argsptr, fd) # fd
         self.ctx.write32(argsptr + 4, argsptr + 12) # data
@@ -221,7 +223,7 @@ class SemihostRequestBuilder:
     def do_write0(self, s):
         argsptr = self.setup_semihost_request(semihost.TARGET_SYS_WRITE0)
 
-        s = bytearray(s + '\\x00')
+        s = bytearray(six.ensure_binary(s) + b'\\x00')
         self.ctx.write_memory_block8(argsptr, s)
 
         was_semihost = run_til_halt(self.tgt, self.semihostagent)
@@ -234,7 +236,7 @@ class SemihostRequestBuilder:
         argsptr = self.setup_semihost_request(semihost.TARGET_SYS_READ)
 
         # Clear read buffer.
-        self.ctx.write_memory_block8(argsptr + 12, bytearray('\\x00') * length)
+        self.ctx.write_memory_block8(argsptr + 12, bytearray(b'\\x00') * length)
 
         self.ctx.write32(argsptr, fd) # fd
         self.ctx.write32(argsptr + 4, argsptr + 12) # ptr
@@ -247,7 +249,7 @@ class SemihostRequestBuilder:
         result = self.ctx.read_core_register('r0')
 
         # Read data put into read buffer.
-        data = str(bytearray(self.tgt.read_memory_block8(argsptr + 12, length - result)))
+        data = byte_list_to_bytes(self.tgt.read_memory_block8(argsptr + 12, length - result))
 
         return result, data
 
@@ -341,11 +343,11 @@ class TestSemihosting:
         assert result == 0
 
     @pytest.mark.parametrize(("writeData", "pos", "readLen", "readResult"), [
-            ("12345678", 0, 8, 0),
-            ("hi", 0, 2, 0),
-            ("hello", 2, 3, 0),
-            ("", 0, 4, 4),
-            ("abcd", -1, 0, 0)
+            (b"12345678", 0, 8, 0),
+            (b"hi", 0, 2, 0),
+            (b"hello", 2, 3, 0),
+            (b"", 0, 4, 4),
+            (b"abcd", -1, 0, 0)
         ])
     def test_file_write_read(self, semihost_builder, delete_testfile, writeData, pos, readLen, readResult):
         fd = semihost_builder.do_open("testfile", 'w+b')
@@ -401,11 +403,11 @@ class TestSemihosting:
         assert console.get_output_data(semihost.STDOUT_FD) == 'this is a string'
 
     @pytest.mark.parametrize(("data", "readlen"), [
-            ("12345678", 8),
-            ("hi", 2),
-            ("hello", 3),
-            ("", 4),
-            ("abcd", 0)
+            (b"12345678", 8),
+            (b"hi", 2),
+            (b"hello", 3),
+            (b"", 4),
+            (b"abcd", 0)
         ])
     def test_console_read(self, semihost_builder, data, readlen):
         console = RecordingSemihostIOHandler()
@@ -471,80 +473,80 @@ class TestSemihosting:
         result = semihost_builder.do_close(fd)
         assert result == 0
 
-@pytest.fixture(scope='function')
-def telnet(request):
-    telnet = semihost.TelnetSemihostIOHandler(4444)
-    def stopit():
-        telnet.stop()
-    request.addfinalizer(stopit)
-    return telnet
-
-@pytest.fixture(scope='function')
-def semihost_telnet_agent(ctx, telnet, request):
-    agent = semihost.SemihostAgent(ctx, console=telnet)
-    def cleanup():
-        agent.cleanup()
-    request.addfinalizer(cleanup)
-    return agent
-
-@pytest.fixture(scope='function')
-def semihost_telnet_builder(tgt, semihost_telnet_agent, ramrgn):
-    return SemihostRequestBuilder(tgt, semihost_telnet_agent, ramrgn)
-
-@pytest.fixture(scope='function')
-def telnet_conn(request):
-    from time import sleep
-    # Sleep for a bit to ensure the semihost telnet server has started up in its own thread.
-    sleep(0.25)
-    telnet = telnetlib.Telnet('localhost', 4444, 10.0)
-    def cleanup():
-        telnet.close()
-    request.addfinalizer(cleanup)
-    return telnet
-
-class TestSemihostingTelnet:
-    def test_connect(self, semihost_telnet_builder, telnet_conn):
-        result = semihost_telnet_builder.do_no_args_call(semihost.TARGET_SYS_ERRNO)
-        assert result == 0
-
-    def test_write(self, semihost_telnet_builder, telnet_conn):
-        result = semihost_telnet_builder.do_write(semihost.STDOUT_FD, 'hello world')
-        assert result == 0
-
-        index, _, text = telnet_conn.expect(['hello world'])
-        assert index != -1
-        assert text == 'hello world'
-
-    def test_writec(self, semihost_telnet_builder, telnet_conn):
-        for c in 'xyzzy':
-            result = semihost_telnet_builder.do_writec(c)
-            assert result == 0
-
-            index, _, text = telnet_conn.expect([c])
-            assert index != -1
-            assert text == c
-
-    def test_write0(self, semihost_telnet_builder, telnet_conn):
-        result = semihost_telnet_builder.do_write0('hello world')
-        assert result == 0
-
-        index, _, text = telnet_conn.expect(['hello world'])
-        assert index != -1
-        assert text == 'hello world'
-
-    def test_read(self, semihost_telnet_builder, telnet_conn):
-        telnet_conn.write('hello world')
-
-        result, data = semihost_telnet_builder.do_read(semihost.STDIN_FD, 11)
-        assert result == 0
-        assert data == 'hello world'
-
-    def test_readc(self, semihost_telnet_builder, telnet_conn):
-        telnet_conn.write('xyz')
-
-        for c in 'xyz':
-            rc = semihost_telnet_builder.do_no_args_call(semihost.TARGET_SYS_READC)
-            assert chr(rc) == c
+# @pytest.fixture(scope='function')
+# def telnet(request):
+#     telnet = semihost.TelnetSemihostIOHandler(4444)
+#     def stopit():
+#         telnet.stop()
+#     request.addfinalizer(stopit)
+#     return telnet
+# 
+# @pytest.fixture(scope='function')
+# def semihost_telnet_agent(ctx, telnet, request):
+#     agent = semihost.SemihostAgent(ctx, console=telnet)
+#     def cleanup():
+#         agent.cleanup()
+#     request.addfinalizer(cleanup)
+#     return agent
+# 
+# @pytest.fixture(scope='function')
+# def semihost_telnet_builder(tgt, semihost_telnet_agent, ramrgn):
+#     return SemihostRequestBuilder(tgt, semihost_telnet_agent, ramrgn)
+# 
+# @pytest.fixture(scope='function')
+# def telnet_conn(request):
+#     from time import sleep
+#     # Sleep for a bit to ensure the semihost telnet server has started up in its own thread.
+#     sleep(0.25)
+#     telnet = telnetlib.Telnet('localhost', 4444, 10.0)
+#     def cleanup():
+#         telnet.close()
+#     request.addfinalizer(cleanup)
+#     return telnet
+# 
+# class TestSemihostingTelnet:
+#     def test_connect(self, semihost_telnet_builder, telnet_conn):
+#         result = semihost_telnet_builder.do_no_args_call(semihost.TARGET_SYS_ERRNO)
+#         assert result == 0
+# 
+#     def test_write(self, semihost_telnet_builder, telnet_conn):
+#         result = semihost_telnet_builder.do_write(semihost.STDOUT_FD, 'hello world')
+#         assert result == 0
+# 
+#         index, _, text = telnet_conn.expect(['hello world'])
+#         assert index != -1
+#         assert text == 'hello world'
+# 
+#     def test_writec(self, semihost_telnet_builder, telnet_conn):
+#         for c in 'xyzzy':
+#             result = semihost_telnet_builder.do_writec(c)
+#             assert result == 0
+# 
+#             index, _, text = telnet_conn.expect([c])
+#             assert index != -1
+#             assert text == c
+# 
+#     def test_write0(self, semihost_telnet_builder, telnet_conn):
+#         result = semihost_telnet_builder.do_write0('hello world')
+#         assert result == 0
+# 
+#         index, _, text = telnet_conn.expect(['hello world'])
+#         assert index != -1
+#         assert text == 'hello world'
+# 
+#     def test_read(self, semihost_telnet_builder, telnet_conn):
+#         telnet_conn.write('hello world')
+# 
+#         result, data = semihost_telnet_builder.do_read(semihost.STDIN_FD, 11)
+#         assert result == 0
+#         assert data == 'hello world'
+# 
+#     def test_readc(self, semihost_telnet_builder, telnet_conn):
+#         telnet_conn.write('xyz')
+# 
+#         for c in 'xyz':
+#             rc = semihost_telnet_builder.do_no_args_call(semihost.TARGET_SYS_READC)
+#             assert chr(rc) == c
 
 class TestSemihostAgent:
     def test_no_io_handler(self, ctx):
@@ -581,7 +583,7 @@ class TestSemihostIOHandlerBase:
         ])
     def test_std_open(self, ctx, ramrgn, ioh, filename, mode, expectedFd):
         handler, agent = ioh
-        ctx.write_memory_block8(ramrgn.start, bytearray(filename) + bytearray('\\x00'))
+        ctx.write_memory_block8(ramrgn.start, bytearray(six.ensure_binary(filename) + b'\\x00'))
         assert handler._std_open(ramrgn.start, len(filename), mode) == (expectedFd, filename)
 
     @pytest.mark.parametrize(("op", "args"), [
['pyocd/debug/semihost.py', 'pyocd/utility/compatibility.py', 'test/unit/conftest.py', 'test/unit/test_semihosting.py']