Add the msvcrt moduleto the default module deny list

Commit https://github.com/python/cpython/commit/880d42a3b247 (first
released in Python 3.8a0) moved an import of msvcrt from an if <win32>
block, into a try/except block. So now the import is tried even on Linux
or MacOS.

https://docs.python.org/3/library/msvcrt.html is a Windows specific
builtin.

docs/changelog.rst

@@ -41,6 +41,7 @@ v0.2.10 (unreleased)
 * :gh:issue:`758` fix initilialisation of callback plugins in test suite, to address a `KeyError` in
   :method:`ansible.plugins.callback.CallbackBase.v2_runner_on_start`
 * :gh:issue:`775` Test with Python 3.9
+* :gh:issue:`775` Add msvcrt to the default module deny list
 
 
 v0.2.9 (2019-11-02)

mitogen/core.py

@@ -1270,6 +1270,10 @@ class Importer(object):
         'builtins',
         '__builtin__',
 
+        # On some Python releases (e.g. 3.8, 3.9) the subprocess module tries
+        # to import of this Windows-only builtin module.
+        'msvcrt',
+
         # Python 2.x module that was renamed to _thread in 3.x.
         # This entry avoids a roundtrip on 2.x -> 3.x.
         'thread',