bpo-32929: Dataclasses: Change the tri-state hash parameter to the boolean unsafe_hash. (GH-5891) (GH-5902)

unsafe_hash=False is now the default. It is the same behavior as the old hash=None parameter. unsafe_hash=True will try to add __hash__. If it already exists, TypeError is raised.
(cherry picked from commit dbf9cff4)
Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>