set default value for matplotlib_backend_chosen

Without this the test for whether importing the plotter chooses
the matplotlib backend fails when matplotlib is not installed.

kwant/tests/test_plotter.py

@@ -33,7 +33,7 @@ try:
 
     from matplotlib import pyplot  # pragma: no flakes
 except ImportError:
-    pass
+    matplotlib_backend_chosen = False
 
 from kwant import plotter