Add mypy to static analyzers

See https://github.com/python/mypy.

We could run the tool on the codebase and fix issues that come up, have it run regularly in a GitHub Actions workflow, or even just add it to Codacy.

The tool is helped by adding "type hints" (see PEP 484: https://peps.python.org/pep-0484), so we might want to consider adding those a little bit at a time. That may be a separate issue.