build: replace codecov with a GitHub action

[Agrendalath]

This:

1. Removes Codecov from Python dependencies and uses a GitHub action instead.
2. Adds CI for testing the Python package.
3. Adds commitlint and automated dependency updates.
4. Replaces a custom PyPI token with the common one.
5. Replaces bleach with bleach[css] to use tinycss2.
6. Removes unused files with requirements.

[Agrendalath]

All changes are used only in the CI, so it's safe to merge once it passes.