CI: add Flake8 and Mypy checks

[piegamesde]

What isn't checked automatically isn't worth the effort.

[hifi]

Flake8 is being run in the pre-commit task which is part of the CI. Mypy isn't enabled until it passes.

[piegamesde]

Oh, I totally missed the pre-commit hook, I'll remove Flake8 then. If you want, I can reopen the pull request against the feature/make-mypy-happy so that you merge that one once the check passes.

[hifi]

The feature branch is badly outdated and probably just needs to be redone completely, unfortunately. It was also a bad attempt at fixing it.

[piegamesde]

Do you want me to give a second try at fixing all the type annotations? Also, why was it a bad attempt?

[hifi]

I was adding more classes to fix typing issues, @tetov did a second run in some of his own branches which was better AFAIK.

Edit: and yes of course if you have time you are more than welcome to try :bow: