Fixed #22447 -- Make sure custom model bases can be migrated.

from django.apps.registry import Apps

from django.db import models

from django.utils import six

from django.utils.encoding import python_2_unicode_compatible

class CustomModelBase(models.base.ModelBase):

    pass

class ModelWithCustomBase(six.with_metaclass(CustomModelBase, models.Model)):

    pass

@python_2_unicode_compatible

class UnicodeModel(models.Model):

    title = models.CharField('ÚÑÍ¢ÓÐÉ', max_length=20, default='“Ðjáñgó”')

from django.db.migrations.state import ProjectState, ModelState, InvalidBasesError

from django.test import TestCase

from .models import ModelWithCustomBase

class StateTests(TestCase):

    """

        project_state.add_model_state(ModelState.from_model(Magazine))

        with self.assertRaises(ValueError):

            rendered_state = project_state.render()

class ModelStateTests(TestCase):

    def test_custom_model_base(self):

        state = ModelState.from_model(ModelWithCustomBase)

        self.assertEqual(state.bases, (models.Model,))