[python] Add type hints for generated classes

[cbornet]

It would be great to generate type hints (PEP484/PEP526) for the python generated class so we have IDE completion and type incompatibility warnings.

Eg for :

message Foo {
  string bar = 1;
}

generate

@overload
class Foo:
  bar = None  # type: str

  def __init__(self, bar=None):  # type: (str) -> None
    pass

Foo = _reflection.GeneratedProtocolMessageType('Foo', (_message.Message,), dict(

Note: this way of writing types works with both python 2 and 3. With python >= 3.6, the Foo class could be documented more nicely.
The type annotations are well handled at least by pycharm.

[melledijkstra]

This would be the best!

[sleshep]

+1

[selimyoussry]

+1, check mypy-protobuf by Dropbox in the meantime

[alkuzman]

+1

[anandolee]

As this future is not critical (not a bug or wrong behavior or relate to performance), protobuf team may not have time for it right now. If anyone is interested and willing to contribute on it, feel free to create a PR and assign to me for review.

Note: some field accept other types, for example bool field accept bool and numbers.Integral

[lexrj]

In response to the note: Python's type hinting allows for nuanced typing to support the indicated case

[hanabi1224]

+1