BUG/DEPR: loc.__setitem__ incorrectly accepting positional slices
- [x] closes #26412
- [x] closes #16121
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I plan to flesh out the tests before this goes in.

cc @toobaz 