[1.6.x] Removed thread customizations of six which are now built-in.

def assertRegex(self, *args, **kwargs):

    return getattr(self, _assertRegex)(*args, **kwargs)

add_move(MovedModule("_dummy_thread", "dummy_thread"))

add_move(MovedModule("_thread", "thread"))

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

    ``testcase.assertRegex`` on Python 3. ``assertRegexpMatches`` still

    exists in current Python 3 versions, but issues a warning.

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

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