[QA] Add/fix type hints to Python code

[elpekenin]

Description

Should help prevent/identify bugs

Ton of work to do, so it's going to take some time, expect it being draft for a good while, open for visibility.

Im (trying) sticking to py3.9 features (eg: list[T] over List[T] but no | over Union), as that's currently the oldest version with support (will be out of life on october, as per PEP 596). We can use pyupgrade in the future to get all/most things automatically improved.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).