[1.8.x] Sorted imports with isort; refs #23860.

Backport of 0ed7d15563 from master

tests/postgres_tests/migrations/0001_setup_extensions.py

@@ -1,7 +1,9 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.contrib.postgres.operations import HStoreExtension, UnaccentExtension
+from django.contrib.postgres.operations import (
+    HStoreExtension, UnaccentExtension,
+)
 from django.db import migrations
 
 

tests/postgres_tests/migrations/0002_create_test_models.py

@@ -1,9 +1,9 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.db import models, migrations
 import django.contrib.postgres.fields
 import django.contrib.postgres.fields.hstore
+from django.db import migrations, models
 
 
 class Migration(migrations.Migration):