Downgrade flake8 from 4.0.1 to 3.9.2

[Kludex]

Flake8 is preventing all the other packages to upgrade, due to:

The conflict is caused by:
    flake8 4.0.1 depends on importlib-metadata<4.3; python_version < "3.8"
    pytest 7.0.1 depends on importlib-metadata>=0.12; python_version < "3.8"
    mkdocs 1.3.0 depends on importlib-metadata>=4.3

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies
To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

I suggest to downgrade it, and give preference to the other packages.

[Kludex]

Thanks @aminalaee !