Reduce the amount of monkeypatching we do

[AlexWaygood]

I have been explicitly warned that the next release of flake8 is likely to break flake8-pyi, due to the amount of monkeypatching we do in this plugin, and a refactoring of flake8 internals that's currently underway. We should probably do something about that.

Ideally, I think we'd file PRs to flake8 and pyflakes so that they become pyi-compatible out of the box, rendering some of the crazy hacks in our code base unnecessary. I'm not really sure where to start, though.

[JelleZijlstra]

Yes, ideally flake8 should have native support for linting pyi files, and then this plugin can focus just on providing pyi-specific error codes, instead of the crazy monkeypatching we do right now.

[AlexWaygood]

fixed by #530