Minor black formatting issue

tutor/utils.py

@@ -25,9 +25,11 @@ def random_string(length):
         [random.choice(string.ascii_letters + string.digits) for _ in range(length)]
     )
 
+
 def list_if(services):
     return json.dumps([service[0] for service in services if service[1]])
 
+
 def common_domain(d1, d2):
     """
     Return the common domain between two domain names.