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

Thanks to cdestigter for the report.

tests/migrations/models.py

@@ -3,9 +3,18 @@
 
 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”')

tests/migrations/test_state.py

@@ -3,6 +3,8 @@
 from django.db.migrations.state import ProjectState, ModelState, InvalidBasesError
 from django.test import TestCase
 
+from .models import ModelWithCustomBase
+
 
 class StateTests(TestCase):
     """
@@ -335,3 +337,9 @@ class Meta:
         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,))