diff --git a/python/requirements.txt b/python/requirements.txt
index 7316d105814..438d97e3ddb 100644
--- a/python/requirements.txt
+++ b/python/requirements.txt
@@ -32,4 +32,7 @@ boto3 == 1.4.4
 # https://github.com/servo/servo/pull/18942
 certifi
 
+# For Python3 compatibility
+six
+
 -e python/tidy