REF/TYP: Rename ABCIndexClass->ABCIndex and use cast
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
@rhshadrach can you merge master yep this looks good (just merge master as indicated) and ping on green. @jreback green. thanks @rhshadrach 

IIRC there was an issue for this? or maybe just a comment To my knowledge this https://github.com/pandas-dev/pandas/pull/38055#issuecomment-733966956