COMPAT: Drop reference to deprecated dateutil.zoneinfo.gettz
- [X] closes #19004

I'm assuming these will be handled by CI:
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Not sure this is necessary, it's a very "behind the scenes" fix, replacing `zoneinfo.gettz` with the superior `tz.gettz()`:

- [ ] whatsnew entry
