Since we started porting to Python 3, our builds on Windows fail sometimes, say 5-10% of the runs, in a bizarre and random manner:

___________ ERROR at setup of testMeshAssemblyPlaneControllerNoBBox ___________
[gw3] win32 -- Python 3.5.3 W:\Miniconda\envs\_sci20-win64g-py35\python.exe
file K:\etk\sci20\source\python\sci20\app\mesh_assembly\plane\_tests\test_mesh_assembly_plane_controller.py, line 83
  def testMeshAssemblyPlaneControllerNoBBox(almost_equal):
E       fixture 'py5' not found

Strangely the "name" being reported is always py and some digits, like py2, py7 or py44.

This always happens when running with xdist and in a random test or contest file, which suggest that the assertion rewriting code might some bug in its locking code when writing the rewritten bytecode to the disk, corrupting it.