'Unregistering a serializer doesn\'t cause the registry to be repopulated. Refs #14823'

def test_unregister(self):

    serializers.unregister_serializer(u'xml')
    serializers.register_serializer(u'json3', u'django.core.serializers.json')
    public_formats = serializers.get_public_serializer_formats()
    self.assertNotIn(u'xml', public_formats)
    self.assertIn(u'json3', public_formats)
