[1.11.x] Sorted imports per isort 4.2.9.

Backport of cde31daf88 from master

tests/model_inheritance/models.py

@@ -16,7 +16,6 @@ from __future__ import unicode_literals
 from django.db import models
 from django.utils.encoding import python_2_unicode_compatible
 
-
 #
 # Abstract base classes
 #