BUG: Series[object].__setitem__(ndarray[M8ns]_with_matching_size)
- [ ] closes #xxxx
- [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

wow this looks user facing can you add a note (and rebase) whatsnew added + greenish