Add type hints

In the ratbag-emu re-write I am using type hints. This provides documentation as well as enables us to do static tests (using mypy). It would be great to have it here too.

type hints cheatsheet: https://mypy.readthedocs.io/en/stable/cheat_sheet_py3.html

typing module documentation: https://docs.python.org/3/library/typing.html

Let me know if this is something we want to have here.