Ensure consistent type definitions in python functions

[ninsbl]

In order to be able to use type checking in IDEs, pre-commit, or CI, it would be good if input variables to python functions could be consistently annotated with variable types. That would also be a general improvement of the developer documentation.

This would be a longer, gradual effort. To start off, it could be clarified in the python committer guidelines, how annotation should be done...

see also post by @echoix in #3374 (comment)

[echoix]

It will be easier to not have to do any workarounds for deprecated syntax if we start it with Python 3.9. Python 3.9 introduced [] in typings, deprecating a lot of special types that worked around.

I'll see if I can extract my proof of concept I did (I have to find the branch and if I had committed or not).