py-grpcio[-tools]: updated to 1.24.3

1.24.3
Add strip_prefix to python protoc plugin and py_grpc_library.
Switch py_proto_library from using src to deps to conform with google3.
Expose local credentials on Python layer.
Make default vtable for pointer argument a constant.
Gracefully handle errors from Future object callbacks.
Gracefully handle exceptions raised by signal handlers on the main thread while unary RPCs are in flight.
Separate py_grpc_library and py_proto_library.
Add wait_for_termination method to grpc.Server.
Add Python Cancellation Example.

(adam)