Using `deepcopy` on an Observable object that referenced another (e.g.
domain name -> resolves to -> IPv4 address) would fail because no
`_valid_refs` were available. Assuming that the references have
already been checked and are valid when using `deepcopy`, we can use a
special value ('*') to skip the valid_refs check.
This will also allow creating new versions of ObservedData objects that
contain related objects, since `new_version` calls `deepcopy`.