BUG: Fix for rolling with uneven nanosecond windows have wrong results
- [x] closes #43997
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Appears there's another merge conflict, sorry.  Thanks @rosagold 