Test with mypy and work on adding type annotations

[dairiki]

I've started playing with adding type annotations and testing with mypy.

I think it will be a while before this is merge-ready, but there's enough here to get mypy to run with meaningful results.

(Bugs have already been discovered, proving that this is probably a worthwhile exercise.)

(Run mypy with tox -e mypy. This requires python>=3.8.)

[yagebu]

Nice :)

One thought: I think we could drop support for Python 3.6 (EOL since December) before adding widespread type annotations, which would allow us to do from __future__ import annotations (postponed evaluation of annotations). That way we could some of the more modern typing features without negative impact on older Python versions, e.g. avoid a lot the imports from typing and use list[str] | None etc.