add more type annotations

In master branch, mypy is integrated into CI, but most of the code does not have type annotations yet.

Hello, I might get arround to adding some type annotations. I annotated a few files just to try it out, and I thought I would ask for guidance before continuing.

This is probably too overkill, right? 😅 (looking at the generics in datastruct.py) Would you prefer this precise approach, or just using Any when it gets too complicated?

@kulisak12 I sometimes rather used Any if it got too complicated. :-)

Also, I guess type annotations are maybe most useful at the places that deal with data that gets persisted into the repository. A small steps approach here is likely better managable than a big global refactoring.