Add `pytest-xdist` #2153

Job	Run time
changes	3s
gatekeeper	6s
env	1s
mac	0s
windows	0s
ubuntu	0s
documentation	0s
scripttest / run	1m 11s
pytest	0s
guitest_nix	0s
coverage / generate and upload	56s
guitest / run (windows-latest)	1m 48s
	4m 5s

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