Deprecating Series.argmin and Series.argmax (#16830)
 - [ ] closes #xxxx
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [x] whatsnew entry

This is one step toward closing issue #16830. After this, I will post another pull request containing the code to implement the expected behavior of argmax and argmin for both Series and DataFrame.