BUG: Series.where casting dt64 to int64
- [x] closes #37682
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This sits on top of #38021
@jreback just rebased with a fix for the unused import in groupby.py @jreback gentle ping; in addition to being a bug this is a blocker for further simplification in blocks merge master on this rebased+green rebased+green let's rebase one more time after your _where change was merged; ping on green ping