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