Fix the object is unhashable when inheriting the class

"ComparableVersionedObject"

In python3, A class that overrides __eq__() and does not define
"__hash__()" will have its __hash__() implicitly set to None.
This will cause the set method to unhashable failed.
https://docs.python.org/3/reference/datamodel.html#object.__hash__

Change-Id: I21685303e9bd233241b7b4216e3feb7bdaecc0a4
Closes-Bug: #1687592

oslo_versionedobjects/base.py

@@ -706,6 +706,9 @@ class ComparableVersionedObject(object):
             return self.obj_to_primitive() == obj.obj_to_primitive()
         return NotImplemented
 
+    def __hash__(self):
+        return super(ComparableVersionedObject, self).__hash__()
+
     def __ne__(self, obj):
         if hasattr(obj, 'obj_to_primitive'):
             return self.obj_to_primitive() != obj.obj_to_primitive()