Pin xdist version on verfided branch.

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
 
 # onnx doesn't currently have a wheel for 3.8
 tf2onnx>=1.5.5;python_version<'3.8'