[Python] Generate trailing underscore for fields that shadow reserved keywords

[noahseger]

What language does this apply to? Python

Describe the problem you are trying to solve.

Fields named, for example from, are inaccessible because they shadow the Python keyword.

https://stackoverflow.com/a/30145419

Describe the solution you'd like Additionally (backwards compatible) generate a field named like from_ in the Python protoc and PYI protoc.

This convention is documented in PEP8:

single_trailingunderscore: used by convention to avoid conflicts with Python keyword, e.g.

https://peps.python.org/pep-0008/#descriptive-naming-styles

Describe alternatives you've considered getattr/setattr hacks.

Additional context N/A

[honglooker]

FYI @anandolee