CI: Attempt to unpin pytest-xdist
- [x] closes #35756

2.1.0 was released yesterday: https://pypi.org/project/pytest-xdist/#history
/azp run <samp>
Azure Pipelines successfully started running 1 pipeline(s).<br>

</samp> restarting CI to check no flakeyness.  Thanks @alimcmaster1 