Flake8 does not support inline comments for any of the keys. + Cleanup some ignored errors/warnings

[misl6]

See:

• I'm temporarily disabling the issue tracker PyCQA/flake8#1760

From https://flake8.pycqa.org/en/latest/user/configuration.html :

Following the recommended settings for Python’s configparser, Flake8 does not support inline comments for any of the keys. So while this is fine:

[flake8]
per-file-ignores =
    # imported but unused
    __init__.py: F401

this is not:

[flake8]
per-file-ignores =
    __init__.py: F401 # imported but unused

Same of kivy/python-for-android#2708

I also took the occasion to do some cleanup.