COMP: Set MPLBACKEND to Qt5Agg in CI testing

To work around the error:

from matplotlib.backends import _macosx
ImportError: Python is not installed as a framework. The Mac OS X backend will not be able to function correctly if Python is not installed as a framework. See the Python documentation for more information on installing Python as a framework on Mac OS X. Please either reinstall Python as a framework, or try one of the other backends. If you are using (Ana)Conda please install python.app and replace the use of 'python' with 'pythonw'. See 'Working with Matplotlib on OSX' in the Matplotlib FAQ for more information.

.github/workflows/build-test.yml

@@ -22,7 +22,7 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         python -m pip install --upgrade setuptools wheel
-        pip install -r requirements.txt -r requirements-dev.txt
+        pip install -r requirements.txt -r requirements-dev.txt PySide2
     - name: Install package and extension
       run: |
         python -m pip install -e .
@@ -32,6 +32,8 @@ jobs:
         python -m pip install jupyterlab
         jupyter labextension install @jupyter-widgets/jupyterlab-manager
         jupyter labextension install ./js
+      env:
+        MPLBACKEND: Qt5Agg
     - name: Lint with flake8
       run: |
         python -m pip install flake8
@@ -43,6 +45,8 @@ jobs:
       run: |
         python -m pip install pytest
         python -m pytest --tb=long --verbose --deselect tests/test_transform_types.py::test_vtkpolydata_to_geometry
+      env:
+        MPLBACKEND: Qt5Agg
     - name: Test package generation and installation
       run: |
         python setup.py sdist