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

Backport of 0ed7d15563 from master

tests/migrations/test_optimizer.py

@@ -1,9 +1,8 @@
 # -*- coding: utf-8 -*-
 
-from django.test import TestCase
+from django.db import migrations, models
 from django.db.migrations.optimizer import MigrationOptimizer
-from django.db import migrations
-from django.db import models
+from django.test import TestCase
 
 from .models import CustomModelBase, EmptyManager