BUG: is_scalar_indexer
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

ATM we go through the wrong path when setting a listlike entry in a single position.

The broader goal: separate out the cases where setitem is inplace vs copying.