Merge pull request #4387 from Unity-Technologies/master-pin-xdist

Pin xdist version.

test_requirements.txt

 # Test-only dependencies should go here, not in setup.py
 pytest>4.0.0,<6.0.0
 pytest-cov==2.6.1
-pytest-xdist
+pytest-xdist==1.34.0
 
 # PyTorch tests are here for the time being, before they are used in the codebase.
 torch>=1.5.0