Add pytest-xdist to dev-requirements

[mhucka]

This allows for parallel pytest execution.

(By itself, this addition to dev-requirements.txt does nothing; the purpose is to make it a separate commit so that the xdist features can be used in updated workflows and test scripts.)

[pavoljuhas]

Consider if the [psutil] extra is truly needed. IIUC, it only comes into play for --numprocesses=logical option which may not get used at all.

[mhucka]

Consider if the [psutil] extra is truly needed. IIUC, it only comes into play for --numprocesses=logical option which may not get used at all.

Huh. I was going by the explanation in the installation instructions, which state:

To use psutil for detection of the number of CPUs available, install the psutil extra:
pip install pytest-xdist[psutil]

Looking at the code, it looks like it defaults to using psutil.cpu_count() if another flag is not given; i.e., the addition is not purely for logical cpu counts.

However, the rest of the function around that line tries a bunch of other approaches if it doesn't have psutil. Thus, there doesn't seem to be a strong reason to include psutil after all, and I agree with you that it can be removed.

Commit 8be2e8a removes it.