run python tests in CI

[moritz-gross]

the file python.yml is analogous to rust.yml.

I also tried running the tests on my Windows PC and encountered 2 problems:

• format of paths to Rules-files was OS-dependent. Now they are type pathlib.Path instead of str.
• tests for terminal output were hardcoded for the console width of my macbook. I added a pytest-fixture for this. I haven't tested CLI output ever before, so likely there's a less clunky way to do this.

[NSoiffer]

It's a good idea to have tests that run automatically.