Misc/NEWS.d/next/Core and Builtins/2018-05-17-13-06-36.bpo-23722.xisqZk.rst

A :exc:`RuntimeError` is now raised when the custom metaclass doesn't
provide the ``__classcell__`` entry in the namespace passed to
``type.__new__``.  A :exc:`DeprecationWarning` was emitted in Python
3.6--3.7.