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

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.

Misc/NEWS.d/next/Library/2018-02-25-13-47-48.bpo-32929.X2gTDH.rst

+Remove the tri-state parameter "hash", and add the boolean "unsafe_hash". If
+unsafe_hash is True, add a __hash__ function, but if a __hash__ exists,
+raise TypeError.  If unsafe_hash is False, add a __hash__ based on the
+values of eq= and frozen=.  The unsafe_hash=False behavior is the same as
+the old hash=None behavior.  unsafe_hash=False is the default, just as
+hash=None used to be.