Cleanup code

[FriedrichFroebel]

Description

This PR performs a lot of cleanup on the existing code, including (but not limited to):

• Use consistent formatting based upon black and ruff.
• Replace flake8 and isort by ruff.
• Resolve linting issues where possible, for example by cleaning up unused imports, checking undefined and unused variables etc. Some TODOs have been added where it has not been clear whether this has worked at all due to undefined variables.
• Re-enable most of the tests decorated with skip after fixing the apparent issues.
• Change test locale to plain English and replace __import__ by importlib. This is required for the conversion tests as the previously used German locale does not define number words at the moment.

How Has This Been Tested?

• The application can start successfully in a local setup.
• The tests are able to run without errors locally.
• All linter findings where reviewed manually.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[github-advanced-security]

CodeQL found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.

[FriedrichFroebel]

Although CI passes, I am going to leave this rather large PR open for now. Feel free to give it a try (consider backing up your database beforehand) and report back when there are any issues with this.

I will have a look at this again later on as well before actually merging. (The CodeQL errors are unrelated and have already existed in the old code.)