specify metaclasses in a py3-compatible way

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

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

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

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

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

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

src/python/grpcio/grpc/framework/base/_ingestion.py

src/python/grpcio/grpc/framework/base/_interfaces.py

src/python/grpcio/grpc/framework/base/_reception.py

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

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

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

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

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

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

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

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

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

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

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