try diff backend to workaround GH/pytest xvfb crash

Author: petercorke

.github/workflows/master.yml

@@ -33,7 +33,7 @@ jobs:
         pip install pytest-xvfb
     - name: Test with pytest
       env:
-        MPLBACKEND: TkAgg
+        MPLBACKEND: Qt5Agg
       run: |
         pytest -s --ignore=W605 --timeout=50 --timeout_method=thread