Fix arithmetic errors with timedelta64 dtypes
Kind of surprising, but I don't see any Issues for these bugs.  Might be because many of them were found after parametrizing arithmetic tests, so instead of Issues we had xfails.

xref #20088 
xref #22163


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