REGR: DataFrame.__setitem__(slice, val) is positional 
- [x] closes #31469 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

The change that this reverts was unintentional (was a code-cleanup intended to remove accessing a private `loc` method).  That said, I do think we should deprecate it. OK, will need a followup issue for that deprecation if you want to push for it.

Let's give @jreback a chance to look, but this should be good to go. This still needs a backport? @MeeseeksDev backport to 1.0.x