Add `pytest-xdist` #2143

Job	Run time
gatekeeper	4s
changes	8s
env	1s
pytest	0s
guitest_nix	0s
mac	0s
ubuntu	0s
windows	0s
documentation	0s
coverage / generate and upload	1m 17s
guitest / run (windows-latest)	2m 9s
scripttest / run	1m 15s
	4m 54s

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