Update mypy typeshed of generated google/protobuf protos in python w/ mypy-protobuf output

[nipunn1313]

Example file to update. There are several within the google/protobuf directory
https://github.com/python/typeshed/blob/master/third_party/2and3/google/protobuf/compiler/plugin_pb2.pyi

Currently, it's a smorgasbord of handwritten files and autogenerated-by-a-very-old-version-of-mypy-protobuf files

Ideally, we can check in a script which takes a version of protoc and a version of mypy-protobuf as input and will autogenerate all the necessary files - making this process repeatable.

[nipunn1313]

This is done! See python/typeshed#4850 and friends. Typeshed has a script which pulls in a version of protoc and a version of mypy-protobuf