TYP: SelectionMixin
- [ ] 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

@simonjayhawkins thoughts on the FrameOrSeries vs FrameOrSeriesUnion that ive ignored here?
@jbrockmendel i've pushed a commit removing the ignores. feel free to revert and do that as a follow-up. Neat thanks yeah thanks @jbrockmendel and @simonjayhawkins this is like magic.