Introduce type checking to codebase

[nijel]

Weblate now requires Python 3.6, so it's now possible to use type hints and introduce type checking to our CI.

My experience with this is exactly zero and our code base is huge, so getting some experienced help with this is welcome.

[nijel]

This is now a work in progress and is expected to take some time.

• There is mypy extra dependency to install type checking depedencies
• We now have CI job running mypy (ignoring the failures for now)
• Parts of the code base have been annotated
• There are still 400+ errors reported by mypy, most of these come from lack of annotations in Weblate or third-party (things like https://github.com/typeddjango/django-stubs/issues/2224 or https://github.com/typeddjango/django-stubs/issues/1514).

Any contribution to this is welcome, just pick one file and enhance it! There is no desire to do everything at once.

[github-actions[bot]]

This issue seems to be a good fit for newbie contributors. You are welcome to contribute to Weblate! Don't hesitate to ask any questions you would have while implementing this.

You can learn about how to get started in our contributors documentation.