ENH: implement Timedelta.__mod__ and __divmod__
This implements exclusively the `__mod__, __rmod__, __divmod__, __rdivmod__` parts of #19365, does _not_ implement any of the bugfixes or tests that got rolled into that PR.  Several tests are xfailed for now, will be fixed after #19738.  These tests will be rebased/moved to test_arithmetic after #19752.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
