API: Index.__cmp__(Series) return NotImplemented
- [x] closes #36759
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Gets rid of an unnecessary special case.

Could do this as a deprecation first.

if this isnt something we want to do, can close Discussed on Nov 2020 dev call, decided to put this off until we have BoolIndex