BUG: Fix length 1 .loc slicing with datetime string GH16071
I've written the test in pytest style since I read that you are moving to this style. If I misunderstood I can add the test to the class. 

- [x] closes #16071 
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry
