Merge pull request #3039 from nathanielmanistaatgoogle/transport-objects

src/python/grpcio/grpc/_links/invocation.py

src/python/grpcio/grpc/_links/service.py

src/python/grpcio/grpc/framework/core/_transmission.py

src/python/grpcio/grpc/framework/interfaces/links/links.py

...cio_test/grpc_test/_links/_lonely_invocation_link_test.py

...python/grpcio_test/grpc_test/_links/_transmission_test.py

...o_test/grpc_test/framework/interfaces/links/test_cases.py

...st/grpc_test/framework/interfaces/links/test_utilities.py