CLN: parametrize test_nat_comparisons
Parametrize ``TestDatetime64SeriesComparison.test_nat_comparisons``
using ``operator`` module.

If this kind of cleanup is considered reasonable,
then I will go through the remaining tests in this test module.

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