API: Ensure DatetimeTZDtype standardizes pytz timezones
- [x] closes #24713
- [x] closes #23815
- [x] closes #20257
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Includes some related-ish timezone documentation issues.