Introduce type annotations for static typing with mypy
#### Type check tools
- [mypy](https://github.com/python/mypy)
- [beartype](https://github.com/beartype/beartype)
- [pyright](https://github.com/Microsoft/pyright) − [post](https://python.developpez.com/actu/253289/Microsoft-devoile-en-open-source-Pyright-un-verificateur-de-type-statique-pour-le-langage-de-programmation-Python/)
- [pytype](https://github.com/google/pytype)
- [pyre-check](https://pyre-check.org/)
- [typecheck-decorator](https://github.com/prechelt/typecheck-decorator)
- [typeguard](https://github.com/agronholm/typeguard)
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## MyPy typing
- [x] #400: Introduce `pre-commit` hook
- [x] Fix existing issues
- [x] Introduce `mypy` job
- [x] Annotate code
- [x] Don’t forget **DeathReaper** on `256_excluded` branch
- [x] Add `mypy` badge
issue