Test with dateutil timezones [Python, guided by maintainer]

[niccokunzmann]

At the moment, we check icalendar with zoneinfo and pytz. We can also make sure that dateutil works.

• [ ] create a TimzoneProvider for dateutil
• [ ] parametrize the tests to work with dateutil (conftest.py)
• [ ] add possible additional tests

[c0d33ngr]

I'll work on this