TYP: Add cast to ABC Index-like types
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

cc @simonjayhawkins 

For the added type-ignore, the attribute `inferred_freq` is added dynamically onto the index instance.