BUG: Timedelta drops decimals if precision is greater than nanoseconds
- [x] closes #36738
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I am not quite sure, if we should round to nanoseconds or cut if off after the 9th decimal. I implemented a simple cut off, but would be happy to change if rounding is better.

I would also add a note to the user guide if the future behavior is clear