BUG: recognize M8[ns, tzstr]
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

ATM `dtype == "M8[ns, UTC]"` fails while `dtype == "datetime64[ns, UTC]"` succeeds