Merge pull request #22452 from lidizheng/fix-call-test

[Aio] Fix call_test: Assert with public API instead of private API

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

@@ -209,7 +209,7 @@ class TestUnaryUnaryCall(_MulticallableTestMixin, AioTestBase):
             grpc.access_token_call_credentials("def"),
         )
         with self.assertRaisesRegex(
-                grpc._cygrpc.UsageError,
+                aio.UsageError,
                 "Call credentials are only valid on secure channels"):
             self._stub.UnaryCall(messages_pb2.SimpleRequest(),
                                  credentials=call_credentials)