BUG: Series constructor drops nanoseconds of Timedelta scalar
- [x] closes #38032
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Well, actually, do this changes in cast.py make sense? cc @jbrockmendel  is #38405 doing the same thing as this? > is #38405 doing the same thing as this?

Yes, I'll close.