Fix Timedelta.__floordiv__, __rfloordiv__
It would not at all surprise me to learn that there are more corner cases that this misses.  Needs some eyeballs.

- [x] closes #18846
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
