Replaced PANDAS_DATETIMEUNIT with NPY_DATETIMEUNIT
progress towards #21852

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

Working on my C-fu as well. There may be a better way to do this (ex: even adding NPY_DATETIMEUNIT to Cython/includes) but figured I'd offer this up for review

@jbrockmendel 