Fix missing Qt on headless CI (#864)

* Fix Qt xcb error as [documented by conda-forge FAQ](https://conda-forge.org/docs/maintainer/maintainer_faq.html#mfaq-qt-load-xcb)

* Remove xvfb and DISPLAY

    Shouldn't be needed with `QT_QPA_PLATFORM=offscreen`?

* Fixes #862

.azure/pipelines.yml

@@ -184,16 +184,6 @@ stages:
           conda env export --name myEnvironment
         displayName: Environment
 
-      - bash: |
-          # lack of display causes tests to abort at VtkViewer
-          # Workaround @ http://docs.enthought.com/mayavi/mayavi/tips.html#rendering-using-the-virtual-framebuffer
-          # and https://github.com/wpilibsuite/RobotBuilder/blob/main/azure-pipelines.yml
-          sudo apt-get update
-          sudo apt-get --yes install xvfb
-          Xvfb :10 &
-        displayName: 'Install and start xvfb'
-        condition: startsWith(variables.image, 'ubuntu')
-
       - bash: |
           source activate myEnvironment
           python setup.py install
@@ -202,7 +192,7 @@ stages:
           FIPY_SOLVERS: $(FIPY_SOLVERS)
           MPIRUN: $(MPIRUN)
           OMP_NUM_THREADS: 1
-          DISPLAY: ':10'
+          QT_QPA_PLATFORM: offscreen
         displayName: Test
 
   - stage: Package
@@ -301,16 +291,6 @@ stages:
         displayName: Build package
         condition: startsWith(variables.image, 'windows')
 
-      - bash: |
-          # lack of display causes tests to abort at VtkViewer
-          # Workaround @ http://docs.enthought.com/mayavi/mayavi/tips.html#rendering-using-the-virtual-framebuffer
-          # and https://github.com/wpilibsuite/RobotBuilder/blob/main/azure-pipelines.yml
-          sudo apt-get update
-          sudo apt-get --yes install xvfb
-          Xvfb :10 &
-        displayName: 'Install and start xvfb'
-        condition: startsWith(variables.image, 'ubuntu')
-
       - bash: |
           source activate myEnvironment
           FIPY_VERSION=`python setup.py --version`
@@ -323,7 +303,7 @@ stages:
           cd ..
           python -c "import fipy; fipy.test()"
         env:
-          DISPLAY: ':10'
+          QT_QPA_PLATFORM: offscreen
         displayName: Test package
         condition: startsWith(variables.image, 'ubuntu')