python: Normalize quotes with Black.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zilencer/migrations/0013_remove_customer_billing_user.py

@@ -6,12 +6,12 @@ from django.db import migrations
 class Migration(migrations.Migration):
 
     dependencies = [
-        ('zilencer', '0012_coupon'),
+        ("zilencer", "0012_coupon"),
     ]
 
     operations = [
         migrations.RemoveField(
-            model_name='customer',
-            name='billing_user',
+            model_name="customer",
+            name="billing_user",
         ),
     ]