Use the correct machanism to ignore test in Windows

src/python/grpcio_tests/commands.py

@@ -161,8 +161,6 @@ class TestGevent(setuptools.Command):
     BANNED_WINDOWS_TESTS = (
         # TODO(https://github.com/grpc/grpc/pull/15411) enable this test
         'unit._dns_resolver_test.DNSResolverTest.test_connect_loopback',
-        # TODO(https://github.com/grpc/grpc/issues/20078) enable this test
-        'unit._local_credentials_test.LocalCredentialsTest.test_local_tcp',
     )
     description = 'run tests with gevent.  Assumes grpc/gevent are installed'
     user_options = []
@@ -189,7 +187,7 @@ class TestGevent(setuptools.Command):
         loader = tests.Loader()
         loader.loadTestsFromNames(['tests'])
         runner = tests.Runner()
-        if sys.platform == 'win32':
+        if os.name == 'nt':
             runner.skip_tests(self.BANNED_TESTS + self.BANNED_WINDOWS_TESTS)
         else:
             runner.skip_tests(self.BANNED_TESTS)

src/python/grpcio_tests/tests/unit/_local_credentials_test.py

@@ -33,6 +33,8 @@ class LocalCredentialsTest(unittest.TestCase):
         server.add_generic_rpc_handlers((_GenericHandler(),))
         return server
 
+    @unittest.skipIf(os.name == 'nt',
+                     'TODO(https://github.com/grpc/grpc/issues/20078)')
     def test_local_tcp(self):
         server_addr = 'localhost:{}'
         channel_creds = grpc.local_channel_credentials(