Improvement: Adding type annotation and add MyPy in CI

[darnuria]

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.

[julien-duponchelle]

Happy to to get this contribution !