Add lint checks to CI (without Stickler CI)

[Peque]

Using flake8 for example.

It seems Stickler CI only checks new changes but does not check old code, so there are pieces that could be cleaned (i.e.: unused parameters/imports/variables).

Also, this would allow for local lint check execution.

[Peque]

Tox could be used as well to ease local vs. CI reproducibility.

[Peque]

I think this issue could have been prevented with lint checks (unused parameters): #1215