always operate on class in super and not on an instance. that's the proper way.

jedi/evaluate/representation.py

@@ -176,7 +176,7 @@ class Instance(use_metaclass(CachedMetaClass, Executable)):
 
     def __getattr__(self, name):
         if name not in ['start_pos', 'end_pos', 'name', 'get_imports',
-                        'doc', 'raw_doc', 'asserts', 'py_bases']:
+                        'doc', 'raw_doc', 'asserts']:
             raise AttributeError("Instance %s: Don't touch this (%s)!"
                                  % (self, name))
         return getattr(self.base, name)

jedi/evaluate/stdlib.py

@@ -100,6 +100,8 @@ def builtins_super(evaluator, obj, params):
         if isinstance(cls, wanted):
             if isinstance(cls, pr.Class):
                 cls = er.Class(evaluator, cls)
+            elif isinstance(cls, er.Instance):
+                cls = cls.base
             su = cls.py_bases()
             if su:
                 return evaluator.execute(su[0])