✅(backend) improve linting

After my first commit of ma contribution, my dear collegue
show me how to improve the linting of a string.
Due to the awesome circle ci, I have to write a long text to explain why I just add a double quote and a return line.
It's wonderful !

Author: Nicolas CAN

src/backend/joanie/tests/core/api/admin/orders/test_refund.py

@@ -328,8 +328,8 @@ def test_api_admin_orders_refund_an_order(self):
             f"Withdrawn on {order.payment_schedule[3]['due_date'].strftime('%m/%d/%Y')}",
             "Canceled",
             "Total €100.00",
-            f"This mail has been sent to {order.owner.email} by Test Catalog \
-                [https://richie.education]",
+            f"This mail has been sent to {order.owner.email}",
+            "by Test Catalog [https://richie.education]",
         ]
 
         self.assertEqual(