Document ``six.assertRaisesRegex``.

    2 and :meth:`~django.utils.datastructures.MultiValueDict.lists()` on

    Python 3.

.. function:: assertRaisesRegex(testcase, *args, **kwargs)

    This replaces ``testcase.assertRaisesRegexp`` on Python 2, and

    ``testcase.assertRaisesRegex`` on Python 3. ``assertRaisesRegexp`` still

    exists in current Python3 versions, but issues a warning.

In addition to six' defaults moves, Django's version provides ``thread`` as

``_thread`` and ``dummy_thread`` as ``_dummy_thread``.