Temporary fix for not working mpl with Qt5

* matplotlib < 2 has got a bug with the Qt5 backend which makes it
unusable, see the issue at https://github.com/matplotlib/matplotlib/pull/6854
* Work around: istall python3-matplotlib-qt4 instead

group_vars/python.yml

@@ -3,7 +3,8 @@ python3pkg:
   - python3
   - python3-setuptools
   - python3-pip
-  - python3-matplotlib-qt5
+  # - python3-matplotlib-qt5  # does not work: https://github.com/matplotlib/matplotlib/pull/6854
+  - python3-matplotlib-qt4
   - python3-ipython
   - python3-numpy
   - python3-scipy