Fix python 3 incompatibility

helios/models.py

@@ -496,7 +496,7 @@ class Election(HeliosModel):
     for eligibility_case in self.eligibility:
       auth_system = eligibility_case['auth_system']
 
-      if not eligibility_case.has_key('constraint'):
+      if not 'constraint' in eligibility_case:
         total_load = False
       else:
         if hasattr(AUTH_SYSTEMS[auth_system], 'can_list_category_members'):