Add mypy to pre-commit

[Hagellach37]

this will help us to improve the code and make use of typed python

[laurentS]

I would also add a mypy run to CI, possibly making it "allowed to fail" at first, but it's a good reminder to run mypy even if you are not using the pre-commit-hook (say you're a new dev, and you missed that part of the setup) :)