TYP: func argument to DataFrame.apply, DataFrame.applymap, core.apply.frame_apply
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Spinning off from #38416, xref https://github.com/pandas-dev/pandas/pull/38416#discussion_r542911035
thanks @arw2019 