python: Sort imports in database migrations.

zerver/migrations/0001_initial.py

@@ -2,16 +2,17 @@
 # Generated by Django 1.11.2 on 2017-06-22 10:22
 
 import bitfield.models
-from django.conf import settings
 import django.contrib.auth.models
 import django.core.validators
-from django.db import migrations, models
 import django.db.models.deletion
 import django.utils.timezone
-import zerver.models
+from django.conf import settings
+from django.db import migrations, models
 from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
 from django.db.migrations.state import StateApps
 
+import zerver.models
+
 def migrate_existing_attachment_data(apps: StateApps,
                                      schema_editor: DatabaseSchemaEditor) -> None:
     Attachment = apps.get_model('zerver', 'Attachment')