BUG: .isin(...) now coerces sets to lists
- [ ] closes #12988
- [ ] tests added, passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] `comp.isin(values)` now coerces `values` from a set into a list. This was failing when `comp` is a datetime dtype and `values` is a set, where `.isin` will try to run `to_datetime(values)` on the unordered set and fail.
