Add `pytest-xdist` #2149

Job	Run time
env	1s
gatekeeper	3s
changes	2s
ubuntu	0s
mac	0s
documentation	0s
windows	0s
scripttest / run	1m 10s
guitest_nix	0s
pytest	0s
guitest / run (windows-latest)	1m 47s
coverage / generate and upload	46s
	3m 49s

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