CLN: remove Index.__inv__ , add Index.__invert__
- [x] closes #22335
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@jreback @jorisvandenbossche suggestions on where to put a test (``pandas/tests/test_arithmetic.py`` would seem the right place, except it only contains tests involving datetimelike) and which version to target (not sure 0.23.5 is OK)?