[1.7.x] Fixed some flake8 errors.

Backport of 9d30412a5a from master

tests/migrations/test_migrations_backwards_deps_1/0001_initial.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.db import migrations, models
+from django.db import migrations
 
 
 class Migration(migrations.Migration):

tests/migrations/test_migrations_backwards_deps_1/0002_second.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.db import migrations, models
+from django.db import migrations
 
 
 class Migration(migrations.Migration):