Fix tzawareness_compat for DatetimeIndex comparisons with NaT
ATM `pd.date_range('2016-01-01', periods=1, tz='US/Pacific') < pd.NaT` raises `TypeError` because `NaT` is tz-naive.  This fixes that.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
