CI: remove stale assignments
Use a third-party GitHub Action to handle the claiming of issues. [See it in action.](https://github.com/afeld/pandas/issues/2#issuecomment-779635007)

It also handles automatically unassigning them after a period of inactivity, which is the more interesting part. The motivation here is that I bet there are a lot of issues where someone has used `take`, but the issue remains open without movement. [By default](https://github.com/JasonEtco/slash-assign-action#options), this Action will remove the assignment after 21 days. Open to suggestions about whether that should be longer or shorter.

- [ ] ~~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~~
