TST: fixtures for timezones
This is a follow up for #20287 

Different tests use different lists of timezones and it's kind of hard to unify all of them. In this PR I tried to create a fixture that covers most frequent cases. Some tests use naive timezone, while others are designed to use only explicit timezones (and fail in case tz is not specified), that's why I created two fixtures to cover these two group of tests. List of timezones also includes one dateutil-timezone. 
 I'm not sure if described approach is correct though so I would appreciate any feedback.

- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
