make iterators python3-compatible

src/python/grpcio/grpc/_adapter/_types.py

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

src/python/grpcio/grpc/framework/crust/_control.py

src/python/grpcio/grpc/framework/face/_control.py

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

src/python/grpcio/grpc/framework/foundation/stream_util.py

src/python/grpcio/tests/interop/methods.py

src/python/grpcio/tests/unit/beta/_beta_features_test.py

...future_invocation_asynchronous_event_service_test_case.py

...ces/face/_future_invocation_asynchronous_event_service.py