REF: make libreduction behavior match _aggregate_series_pure_python
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

nice! There's more coming up in this sequence of PRs.  At some point further simplification will need decisions on #41090 and #28900.