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

def test_unregister(self):

    serializers.unregister_serializer('xml')
    serializers.register_serializer('json3', 'django.core.serializers.json')
    public_formats = serializers.get_public_serializer_formats()
    self.assertFalse(('xml' in public_formats))
    self.assertTrue(('json3' in public_formats))
