[WIP] Add typechecking using mypy

[jderuiter]

• Add mypy checks to the test script
• Add mypy checks to CI
• Add type annotations

Fixes #2722

[mouse-reeve]

I merged in main to fix the failing Prettier workflow which wasn't related to your work. And thank you for opening this PR!

[jderuiter]

Thank you! I've been adding type annotations to the code, but I'm wondering what the best way forward is. I'm not familiar enough with the code base and Django internals to always know the intent of the code to add the correct types. For now I just added questions I had in TODOs. What do you think would be the best way to do this?

[mouse-reeve]

I think it may be a good idea to break this into a series of pull requests, doing one module at a time. That way there's less risk of merge conflicts, and we can address any fixes in more manageable chunks. I'm on my phone right now but when I'm back at my computer I can take a closer look at the custom types. But I think starting with one or two relatively simple files might be a nice way to get going

[jderuiter]

Sounds good. I created a smaller pull request with just the mypy tooling and some first annotations ( #2916). Once that is merged we can start looking at the other annotations I already added.