TST/BUG: test unsortable in safe_sort and Categorical (GH13714)
- [x] closes #13714
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry
- Change test for Categorical.from_array (`test_constructor_unsortable`)
- Add test for safe_sort
- Change 2.7 build to latest numpy

---

`safe_sort` always raises on a mixed int-datetime array. This is the expected behaviour. (But, for numpy >= 1.10 & python2, I still have some doubts whether it's not a manifestation of an undefined behaviour.)
