Improvement: Adding type annotation and add MyPy in CI

Hello pretty much in the Title, if pertinent I can try to work on that modestly.

- Add types annotations (try to avoid requirement on too-high python version)
- Add mypy in CI to check theses (i am new with github action)

I know that some function signature would be hairy but it's okay, will help discovering possibility I think, right now it's often necessary to jump into the source to know what exactly a function accept.

Start by surveying the repository's Python modules and current CI configuration to determine the annotation scope and the appropriate MyPy check. The work is done when relevant function signatures are annotated without requiring an unnecessarily new Python version and CI checks those annotations with MyPy.

Written by the indexing model from the issue text.