Better typing support

[bhirsz]

Our code is partly typed but we miss most of the typing. We should add typing information to our code (or at least to most of it).

[borutzki]

Do you see it as a project to be done in one PR or split into multiple ones?

Also, I am quite new here. Is there already a type checker selected for this project? Which way of adding type hints should be used?

I'm considering doing it as a side-job but probably it's better first to ask about that and only then start working on a whole repo.

[bhirsz]

Doesn't matter but considering the scope, it can be split into multiple parts so it's easier.

We don't use any, anything popular is fine - like mypy etc. It depends, we could start by fixing issues (like run mypy/any similar tool, catch problems and fix them) or add typing where it's needed the most. We need to have performance in mind - if we need to import whole new package to add typing, it's better to use "type in string" with flag that only import such package when typing check is performed. But other than that I don't have any special condition in mind and it certainly can be done by anyone who is willing to :)