This is the mirror image of #2170.

### Checklists

<!-- Note on checklists:
     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
     if some checkboxes are left unmarked, we might assume that your PR
     is not ready to be merged and we might keep you waiting -->

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests
- [x] Add new / update outdated documentation

Add `default_converter` argument to `to_js` and `toJs`. This is the mirror image of #2170.

### Checklists

<!-- Note on checklists:
     If you think some of these steps are not necessary for your PR,
     just remove those checkboxes, or mark them as checked. Otherwise,
     if some checkboxes are left unmarked, we might assume that your PR
     is not ready to be merged and we might keep you waiting -->

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests
- [x] Add new / update outdated documentation

From a4aec920c76ebcb8360350ff0046c7a0c74c0cf2 Mon Sep 17 00:00:00 2001
From: Hood <hood@mit.edu>
Date: Thu, 24 Jun 2021 14:55:10 -0700
Subject: [PATCH] xfail core ctypes tests that don't work

PyCode_SetExtra doesn't work and ctypes doesn't seem to work with variadic functions including PyUnicode_FromFormat/
---
 Lib/test/test_code.py    | 7 +++++--
 Lib/test/test_unicode.py | 1 +
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/Lib/test/test_code.py b/Lib/test/test_code.py
index ac3dde7..d91a350 100644
--- a/Lib/test/test_code.py
+++ b/Lib/test/test_code.py
@@ -389,6 +389,7 @@ if check_impl_detail(cpython=True) and ctypes is not None:
                               ctypes.c_voidp(100)), 0)
 
         def test_free_called(self):
+            raise unittest.SkipTest("PyCode_SetExtra is broken")
             # Verify that the provided free function gets invoked
             # when the code object is cleaned up.
             f = self.get_func()
@@ -398,6 +399,7 @@ if check_impl_detail(cpython=True) and ctypes is not None:
             self.assertEqual(LAST_FREED, 100)
 
         def test_get_set(self):
+            raise unittest.SkipTest("PyCode_SetExtra is broken")
             # Test basic get/set round tripping.
             f = self.get_func()
 
@@ -414,6 +416,7 @@ if check_impl_detail(cpython=True) and ctypes is not None:
             del f
 
         def test_free_different_thread(self):
+            raise unittest.SkipTest("Requires threading")
             # Freeing a code object on a different thread then
             # where the co_extra was set should be safe.
             f = self.get_func()
@@ -438,8 +441,8 @@ def test_main(verbose=None):
     from test import test_code
     run_doctest(test_code, verbose)
     tests = [CodeTest, CodeConstsTest, CodeWeakRefTest]
-    if check_impl_detail(cpython=True) and ctypes is not None:
-        tests.append(CoExtra)
+    # if check_impl_detail(cpython=True) and ctypes is not None:
+    #     tests.append(CoExtra)
     run_unittest(*tests)
 
 if __name__ == "__main__":

From a4aec920c76ebcb8360350ff0046c7a0c74c0cf2 Mon Sep 17 00:00:00 2001
From: Hood <hood@mit.edu>
Date: Thu, 24 Jun 2021 14:55:10 -0700
Subject: [PATCH] xfail core ctypes tests that don't work

PyCode_SetExtra doesn't work and ctypes doesn't seem to work with variadic functions including PyUnicode_FromFormat/
---
 Lib/test/test_code.py    | 7 +++++--
 Lib/test/test_unicode.py | 1 +
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/Lib/test/test_code.py b/Lib/test/test_code.py
index ac3dde7..d91a350 100644
--- a/Lib/test/test_code.py
+++ b/Lib/test/test_code.py
@@ -389,6 +389,7 @@ if check_impl_detail(cpython=True) and ctypes is not None:
                               ctypes.c_voidp(100)), 0)
 
         def test_free_called(self):
+            raise unittest.SkipTest("PyCode_SetExtra is broken")
             # Verify that the provided free function gets invoked
             # when the code object is cleaned up.
             f = self.get_func()
@@ -398,6 +399,7 @@ if check_impl_detail(cpython=True) and ctypes is not None:
             self.assertEqual(LAST_FREED, 100)
 
         def test_get_set(self):
+            raise unittest.SkipTest("PyCode_SetExtra is broken")
             # Test basic get/set round tripping.
             f = self.get_func()
 
@@ -414,6 +416,7 @@ if check_impl_detail(cpython=True) and ctypes is not None:
             del f
 
         def test_free_different_thread(self):
+            raise unittest.SkipTest("Requires threading")
             # Freeing a code object on a different thread then
             # where the co_extra was set should be safe.
             f = self.get_func()
@@ -438,8 +441,8 @@ def test_main(verbose=None):
     from test import test_code
     run_doctest(test_code, verbose)
     tests = [CodeTest, CodeConstsTest, CodeWeakRefTest]
-    if check_impl_detail(cpython=True) and ctypes is not None:
-        tests.append(CoExtra)
+    # if check_impl_detail(cpython=True) and ctypes is not None:
+    #     tests.append(CoExtra)
     run_unittest(*tests)
 
 if __name__ == "__main__":
