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

Doesn't appear to affect anything in master, but stumbled on it when implementing the PR do always return views when indexing on columns.