Fixes a Python 3.x regression introduced in a19e9d80

import io

from django.core import management

from django.test import TestCase

from django.utils import six

class ModelValidationTest(TestCase):

        # Validation Tests:

        #   * choices= Iterable of Iterables

        #       See: https://code.djangoproject.com/ticket/20430

        management.call_command("validate", stdout=io.BytesIO())

        management.call_command("validate", stdout=six.StringIO())