[0.27] 'JLinkProbe' object has no attribute '_lock'
Using version 0.27 and JLink, session.open() throws the error below. The previous version was working fine for me.
 
`with ConnectHelper.session_with_chosen_probe() as session:`

```
Traceback (most recent call last):
  -truncated-
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\core\\session.py", line 302, in __enter__
    self.open()
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\core\\session.py", line 420, in open
    self._board.init()
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\board\\board.py", line 85, in init
    self.target.init()
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\core\\coresight_target.py", line 160, in init
    seq.invoke()
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\utility\\sequencer.py", line 213, in invoke
    resultSequence.invoke()
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\utility\\sequencer.py", line 208, in invoke
    resultSequence = call()
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\coresight\\dap.py", line 359, in power_up_debug
    self.write_reg(DP_CTRL_STAT, CSYSPWRUPREQ | CDBGPWRUPREQ | MASKLANE | TRNNORMAL)
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\coresight\\dap.py", line 348, in write_reg
    self.write_dp(addr, data)
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\coresight\\dap.py", line 528, in write_dp
    did_lock = self._set_dpbanksel(addr, True)
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\coresight\\dap.py", line 480, in _set_dpbanksel
    self.lock()
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\coresight\\dap.py", line 252, in lock
    self.probe.lock()
  File "C:\\xxx\\pycharm\\lib\\site-packages\\pyocd\\probe\\debug_probe.py", line 173, in lock
    self._lock.acquire()
AttributeError: 'JLinkProbe' object has no attribute '_lock'
```
diff --git a/pyocd/probe/jlink_probe.py b/pyocd/probe/jlink_probe.py
index d5957cd0..ef1e7ef6 100644
--- a/pyocd/probe/jlink_probe.py
+++ b/pyocd/probe/jlink_probe.py
@@ -78,6 +78,7 @@ class JLinkProbe(DebugProbe):
             six.raise_from(cls._convert_exception(exc), exc)
 
     def __init__(self, serial_number):
+        super(JLinkProbe, self).__init__()
         self._link = self._get_jlink()
         if self._link is None:
             raise exceptions.ProbeError("unable to open JLink DLL")
['pyocd/probe/jlink_probe.py']