Random GitHub Actions test failures

[jpgill86]

Every once in a while, seemingly at random, we get GitHub Actions test failures with inscrutable segmentation faults, or errors like this one:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/runner/work/ephyviewer/ephyviewer/ephyviewer/tests/testing_tools.py", line 9, in <module>
    import matplotlib.pyplot as plt
  File "/opt/hostedtoolcache/Python/3.9.6/x64/lib/python3.9/site-packages/shiboken2/files.dir/shibokensupport/__feature__.py", line 142, in _import
    return original_import(name, *args, **kwargs)
  File "/opt/hostedtoolcache/Python/3.9.6/x64/lib/python3.9/site-packages/matplotlib/pyplot.py", line 2500, in <module>
    switch_backend(rcParams["backend"])
  File "/opt/hostedtoolcache/Python/3.9.6/x64/lib/python3.9/site-packages/matplotlib/pyplot.py", line 285, in switch_backend
    raise ImportError(
ImportError: Cannot load backend 'Qt5Agg' which requires the 'qt5' interactive framework, as 'headless' is currently running

If the test suite is rerun, it passes. This is strange and annoying.