TO deal with Python3 returning not list but dict_items.

Author: Roland Hedberg

src/saml2/mdstore.py

@@ -305,7 +305,7 @@ def attribute_requirement(self, entity_id, index=None):
         raise NotImplementedError
 
     def dumps(self):
-        return json.dumps(self.items(), indent=2)
+        return json.dumps(list(self.items()), indent=2)
 
     def with_descriptor(self, descriptor):
         '''