* Add script to generate protoc stubs using mypy-protobuf generated stubs
* Use generate_proto_stubs to generate stubs for protobuf 3.14.0
* Skip _pb2.pyi from flake8,black,isort,pytype
Fixes#4288.
- Default imports to THIRD_PARTY, so in effect we merge the FIRST_PARTY and THIRD_PARTY stubs. This means import order is no longer affected by whether typing_extensions is installed locally.
- Treat typing_extensions, _typeshed and some others as standard library modules.
Note that isort master is very different from the latest release; we'll have to do something
different if and when the next isort release comes out.
