BUG: partial-timestamp slicing near the end of year/quarter/month
I'm fairly confident we can refactor this to be a lot less verbose, but will do that separately from the bugfix.

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