Enforce PEP 570-style positional only syntax

[srittau]

flake8-pyi should forbid old-style positional only syntax. I.e., instead of def foo(__bar): ... the following should be used: def foo(bar, /): ....

It's too early to implement this at this moment, but I opened this issue so we remember it. See python/typeshed#4972 for current type checker support.

[AlexWaygood]

I really wish we could implement this feature now ☹ I find the __ syntax incredibly ugly, and it made the stubs look so weird to me when I first started contributing to typeshed (just because of how different it looks to anything you see in normal Python).