BUG: Change IntervalDtype.kind from None to "O"
- [X] closes #30568
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

This is partially blocking #28399 as tests are failing there due to `kind` being `None`.

cc @TomAugspurger 
