BUG: DataFrame reductions inconsistent with Series counterparts
- [x] closes #36076
- [x] closes #28949
- [x] closes #21020
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

#28948 is an incorrect ref (top of PR)