matplotlib is used in tests

packages/py-pynn/package.py

@@ -57,6 +57,7 @@ class PyPynn(PythonPackage):
     depends_on('py-brian2',             type=('run', 'test'))
 
     depends_on('py-mock@1.0:',          type='test')
+    depends_on('py-matplotlib',         type='test')
     depends_on("py-pytest",             type='test', when="@0.11.0:")
 
     patch('pynn-0.9.6-python3.patch', when='@0.9.6 ^python@3:')