Improve infrastructure/style

[lang-m]

I had another quick look at the repository. Here is a list of further improvements we should make.

@thomasschrefl I have added you to a few that you could do with the agent. I'll do some of the smaller things.

• docstrings must not contain types (that's covered by the type annotations) @thomasschrefl
• A few test_* files are still in src @thomasschrefl

Everyone, please feel free to edit this issue to add additional items to the list.