Misc improvements

[dimbleby]

A miscellany of small improvements, which would be five separate pull requests if I were less lazy.

Ask me to split it up if that sort of thing matters to you - or if there are commits in here that you do want mixed up with commits that you do not want.

• make the mypy configuration a little bit stricter (and prettier)
• fix some mis-initializations in test code
• modernize pyproject.toml to PEP621 style
• add incomplete type annotations to the unit tests
  • just enough to stop my editor from marking whole functions red, which was the motivation...
• fix coverage report so that it does only gives statistics for the source code, not for the tests

[pappasam]

🚀 Thanks, for the cleanup, lgtm!