Fix old python super

CHANGELOG

@@ -1,5 +1,5 @@
 V 2.0.0 UNRELEASED
-	Rework the ghost mechanism to come back to a more object oriented behavior, storing all state in a state object which is created every render.
+	Rework the ghost mechanism to come back to a more object oriented behavior, storing all state in a state object which is created on every render. (#161)
 	Refactor maps
 	Add world continents
 	Add swiss cantons map (thanks sergedroz)

pygal/graph/dot.py

@@ -96,7 +96,7 @@ class Dot(Graph):
     @cached_property
     def _values(self):
         """Getter for series values (flattened)"""
-        return [abs(val) for val in super()._values if val != 0]
+        return [abs(val) for val in super(Dot, self)._values if val != 0]
 
     @cached_property
     def _max(self):