Add type hints

It would be great to have type hints. I started adding them in my local version, I can open a PR when more significant work has been done, but I wanted to open this issue for tracking.

No files, tests, or entry points are named. Start by mapping the repository's Python modules and confirming the intended scope and conventions for type hints with the maintainer; done should be defined as an agreed scope with the type hints applied consistently and validated by the project's existing checks.

Written by the indexing model from the issue text.