Attempt to fix all CI failures:

* Fixes the Windows Cython obsolate issue
* Fixes the post fork threading issue

src/python/grpcio/grpc/_cython/_cygrpc/aio/grpc_aio.pxd.pxi

@@ -13,6 +13,7 @@
 # limitations under the License.
 # distutils: language=c++
 
+
 cdef extern from "src/core/lib/iomgr/timer_manager.h":
   void grpc_timer_manager_set_threading(bint enabled);
 

src/python/grpcio/grpc/_cython/_cygrpc/fork_posix.pyx.pxi

@@ -32,6 +32,7 @@ _TRUE_VALUES = ['yes',  'Yes',  'YES', 'true', 'True', 'TRUE', '1']
 # must  not block and should execute quickly.
 #
 # This flag is not supported on Windows.
+# This flag is also not supported for non-native IO manager.
 _GRPC_ENABLE_FORK_SUPPORT = (
     os.environ.get('GRPC_ENABLE_FORK_SUPPORT', '0')
         .lower() in _TRUE_VALUES)

src/python/grpcio_tests/tests/_runner.py

@@ -15,7 +15,6 @@
 from __future__ import absolute_import
 
 import collections
-import multiprocessing
 import os
 import select
 import signal

src/python/grpcio_tests/tests_aio/unit/channel_test.py

@@ -14,6 +14,7 @@
 """Tests behavior of the grpc.aio.Channel class."""
 
 import logging
+import threading
 import unittest
 
 import grpc

tools/run_tests/artifacts/build_artifact_python.bat

@@ -18,6 +18,7 @@ set PATH=C:\%1;C:\%1\scripts;C:\msys64\mingw%2\bin;C:\tools\msys64\mingw%2\bin;%
 python -m pip install --upgrade six
 @rem some artifacts are broken for setuptools 38.5.0. See https://github.com/grpc/grpc/issues/14317
 python -m pip install --upgrade setuptools==38.2.4
+python -m pip install --upgrade cython
 python -m pip install -rrequirements.txt
 
 set GRPC_PYTHON_BUILD_WITH_CYTHON=1

tools/run_tests/run_tests.py

@@ -727,13 +727,17 @@ class PythonLanguage(object):
                 self.args.iomgr_platform]) as tests_json_file:
             tests_json = json.load(tests_json_file)
         environment = dict(_FORCE_ENVIRON_FOR_WRAPPERS)
+        # NOTE(lidiz) Fork handlers is not designed for non-native IO manager.
+        # It has a side-effect that overrides threading settings in C-Core.
+        if args.iomgr_platform != 'native':
+            environment['GRPC_ENABLE_FORK_SUPPORT'] = '0'
         return [
             self.config.job_spec(
                 config.run,
                 timeout_seconds=5 * 60,
                 environ=dict(
-                    list(environment.items()) + [(
-                        'GRPC_PYTHON_TESTRUNNER_FILTER', str(suite_name))]),
+                    GRPC_PYTHON_TESTRUNNER_FILTER=str(suite_name),
+                    **environment),
                 shortname='%s.%s.%s' %
                 (config.name, self._TEST_FOLDER[self.args.iomgr_platform],
                  suite_name),