Tests: Disable matplotlib tests for lab 4

The matplotlib test loads all figures at once, but for lab they have to
be loaded and tested when in view. We disable this test for lab 4 and
put this as task for later. See issue #52.

tests/test_widgets.py

@@ -589,6 +589,7 @@ def test_widget_answer(self, selenium_driver):
         ("tests/notebooks/widget_cue_figure-inline.ipynb", "inline"),
     ],
 )
+@pytest.mark.matplotlib
 def test_widget_figure(selenium_driver, nb_filename, mpl_backend):
     """
     We separate the widget figure tests for different backends to different files

tox.ini

@@ -73,7 +73,7 @@ deps =
 commands =
     # converts the python files to ipython notebooks
     jupytext tests/notebooks/*.py --to ipynb
-    pytest {posargs:-v} --driver Firefox
+    pytest {posargs:-v} -m "not matplotlib" --driver Firefox
 
 [testenv:coverage]
 # We do coverage in a separate environment that skips the selenium tests but