bpo-13601: always use line-buffering for sys.stderr (GH-17646)

diff --git a/Misc/ACKS b/Misc/ACKS
index 5ecbac1..d3e683d 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1510,6 +1510,7 @@
 Nick Seidenman
 Michael Seifert
 Žiga Seilnacht
+Jendrik Seipp
 Michael Selik
 Yury Selivanov
 Fred Sells