Setup flake8-pyi to lint stubs

[sobolevn]

It will enforce ... instead of None, find useless nodes for annotations, etc.
https://github.com/ambv/flake8-pyi

[hoefling]

@sobolevn does the decision on not using flake8-pyi still hold? (If I am reading #132 correctly) IMO making the stubs compliant to PEP 585, PEP 604, PEP 613 & friends would improve stubs quality, flake8-pyi also can prevent commits containing unused imports etc etc.

[sobolevn]

There are so many violations out there.

If you have free time to introduce flake8-pyi and refactor all existing problems - you are more than welcome :)

[hoefling]

@sobolevn let me give it a try then. Most of the findings can be eliminated in an automated fashion - I already did this for PEP 604 imports in typeshed in python/typeshed#5564 and python/typeshed#5915, the scripts should be still functional.

[sobolevn]

Thank you! This will impove our code quality :)

[intgr]

If you're in for an extra challenge, you can do the same to https://github.com/typeddjango/djangorestframework-stubs repository too. :)

[hoefling]

@intgr gladly, but let's discuss #1253 first, I can then apply the same rules for DRF stubs