Added stub release notes for 1.6.6.
Fixed #16245 -- Included traceback in send_robust()'s response

Exceptions from the (receiver, exception) tuples returned by
``send_robust()`` now have always their traceback attached as their
``__traceback__`` argument.
Fixed jenkins breakage, apparently migrations need a models module.

Workaround for now, Andrew will look into it.
Fixed #22634 -- Changed Session and SessionStore to be easily extendable

Introduced abstract base model for Session, and properties enabling
the developer to override session model class used by session store
and the session store class used by the model.