BUG: Bug in loc did not change dtype when complete column was assigned
- [x] closes #20635
- [x] closes #20511
- [x] closes #27583
- [x] closes #33198
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
