'MultiWidget should define __deepcopy__() (#12048).'

def test_deepcopy(self):

    w1 = DeepCopyWidget(choices=[1, 2, 3])
    w2 = copy.deepcopy(w1)
    w2.choices = [4, 5, 6]
    self.assertEqual(w1.choices, [1, 2, 3])
