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
Hello @fjetter! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/dtypes/cast.py`](https://github.com/fjetter/pandas/blob/dece94e2c852d6119a6c5731934dfa9cb38af339/pandas/core/dtypes/cast.py) !

 i think this is covered by #22699 already cc @TomAugspurger  #22699 is silencing it for now. https://github.com/pandas-dev/pandas/pull/22699/files#diff-3bbe4551f20de6060dce38a95d0adc80R961 is the relevant line. As far as testing goes, I don't think we need a new test, as this would have failed if #22699 were in already.

I don't think this needs to go in 0.23.5. closing as stale. if you'd like to continue, pls ping.