BUG: Fix NaT +/- DTA/TDA
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

After this bugfix, we can change all existing uses of `dispatch_to_index_op` in `core.ops.__init__` to use `dispatch_to_extension_op`.  Once we do that, we can collapse ~50 lines of typechecking-dispatching code to all use `dispatch_to_extension_op`