TYP: testing.pyi
- [ ] 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

gentle ping @WillAyd, annotations PRs are piling up and becoming blockers (albeit mostly for further annotations, so no real hurry) thanks @jbrockmendel  booyah!   more of these .pyi files are waiting in the wings once the current crop of TYP PRs go through