DEPR: allowing subclass-specific keywords in pd.Index.__new__
- [x] closes #21311, closes #14093, closes #22315, closes #26974
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

can you merge master once more. i realized that the period keyword removal was merged *before* this somethings still failing in precommit nice!