Resolves Issue 21344: provide a timedelta in a non-string format
- [x] closes #21344
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This extends the API - and there is a judgement call about how to treat nanosecond-resolution pd.Timedelta() objects in the context of datetime.timedelta() objects.
