Python 3.9 support

[gsauthof]

Running the asynctest unittests under Python 3.9 yields a bunch of test case failures.

Example:

======================================================================
ERROR: test_patch_generator_with_multiple_scopes (test.Test_patch_decorator_coroutine_or_generator_scope)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/builddir/build/BUILD/asynctest-0.13.0/test/test_mock.py", line 1650, in test_patch_generator_with_multiple_scopes
    self.assertEqual((True, True), next(gen))
  File "/builddir/build/BUILD/asynctest-0.13.0/asynctest/mock.py", line 995, in patched_generator
    return (yield from patched_factory(*args, **kwargs))
  File "/builddir/build/BUILD/asynctest-0.13.0/asynctest/mock.py", line 985, in patched_factory
    patching.__exit__(*exc_info)
  File "/usr/lib/python3.9/unittest/mock.py", line 1536, in __exit__
    return exit_stack.__exit__(*exc_info)
  File "/usr/lib/python3.9/contextlib.py", line 471, in __exit__
    received_exc = exc_details[0] is not None
IndexError: tuple index out of range
======================================================================
ERROR: test_deactivate_patch_when_generator_exec_fails (test.Test_patch_decorator_coroutine_or_generator_scope_GLOBAL)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/builddir/build/BUILD/asynctest-0.13.0/test/test_mock.py", line 1739, in test_deactivate_patch_when_generator_exec_fails
    self._test_deactivate_patch_when_generator_exec_fails(asynctest.GLOBAL)
  File "/builddir/build/BUILD/asynctest-0.13.0/test/test_mock.py", line 1348, in _test_deactivate_patch_when_generator_exec_fails
    run_coroutine(tester())
  File "/builddir/build/BUILD/asynctest-0.13.0/test/utils.py", line 9, in run_coroutine
    return loop.run_until_complete(coroutine)
  File "/usr/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/builddir/build/BUILD/asynctest-0.13.0/test/test_mock.py", line 1341, in tester
    yield from a_coroutine()
  File "/builddir/build/BUILD/asynctest-0.13.0/asynctest/mock.py", line 995, in patched_generator
    return (yield from patched_factory(*args, **kwargs))
  File "/builddir/build/BUILD/asynctest-0.13.0/asynctest/mock.py", line 1044, in __next__
    self._stop_global_patchings()
  File "/builddir/build/BUILD/asynctest-0.13.0/asynctest/mock.py", line 1032, in _stop_global_patchings
    if _is_started(patching):
  File "/builddir/build/BUILD/asynctest-0.13.0/asynctest/mock.py", line 124, in _is_started
    return unittest.mock._is_started(patching)
AttributeError: module 'unittest.mock' has no attribute '_is_started'
-------------------- >> begin captured logging << --------------------
asyncio: DEBUG: Using selector: EpollSelector
--------------------- >> end captured logging << ---------------------
[..]
======================================================================
FAIL: test_patch_as_decorator_uses_CoroutineMock_on_async_coroutine_function (test.test_mock.Test_patch)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/builddir/build/BUILD/asynctest-0.13.0/test/test_mock.py", line 940, in test_patch_as_decorator_uses_CoroutineMock_on_async_coroutine_function
    test_mock_coroutine()
  File "/usr/lib/python3.9/unittest/mock.py", line 1337, in patched
    return func(*newargs, **newkeywargs)
  File "/builddir/build/BUILD/asynctest-0.13.0/test/test_mock.py", line 937, in test_mock_coroutine
    self.assertIsInstance(mock, asynctest.mock.CoroutineMock)
AssertionError: <MagicMock name='an_async_coroutine' id='4125941040'> is not an instance of <class 'asynctest.mock.CoroutineMock'>

See https://kojipkgs.fedoraproject.org//work/tasks/8916/45078916/build.log for the full log.

The _is_started error is already reported in #149.

[gsauthof]

Related: Currently, asynctest doesn't support Python 3.8, either, cf. #132 and #144