Install matplotlib for brille test

This shouldn't be necessary but might help us figure out why it is failing.

tox.ini

@@ -70,7 +70,7 @@ commands_pre =
     python -m pip install \
         --upgrade \
         --upgrade-strategy eager \
-        '{toxinidir}[brille]'
+        '{toxinidir}[brille,matplotlib]'
 commands = {[testenv]test_command} --cov -m "brille and not multiple_extras"
 
 # Run remaining tests that require multiple extras