Merge pull request #7001 from kpayson64/python_cleanup_thread

src/python/grpcio/grpc/_channel.py

src/python/grpcio/grpc/beta/_server_adaptations.py

src/python/grpcio/tests/tests.json

src/python/grpcio/tests/unit/_exit_scenarios.py

src/python/grpcio/tests/unit/_exit_test.py