diff --git a/IPython/html/tests/launchnotebook.py b/IPython/html/tests/launchnotebook.py
index 0b4f5d4..c9a5ee8 100644
--- a/IPython/html/tests/launchnotebook.py
+++ b/IPython/html/tests/launchnotebook.py
@@ -16,6 +16,8 @@ from IPython.utils.tempdir import TemporaryDirectory
 MAX_WAITTIME = 30   # seconds to wait for notebook server to start
 POLL_INTERVAL = 0.1 # time between attempts
 
+# TimeoutError is a builtin on Python 3. This can be removed when we stop
+# supporting Python 2.
 class TimeoutError(Exception):
     pass