DEPR/BUG: fix Series.argmin/max
 - [x] closes #16830
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [ ] whatsnew entry

This PR actually implements the proper behavior for argmax() and argmin() for both Series and DataFrame. We will have to wait to rebase this onto master until after a new release.