BUG: fix numpy deprecation warning for scalar datetime64 values
This should fix the DeprecationWarning described in #22741 . Not sure if it's worth adding a test. What's the policy on things like these?

- [x] closes #22741
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry