bpo-13601: Mention stderr's line buffering in What's New (GH-20168)

diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
index 479c33b4a7fa1c3c6c559f1b88f09f08316b2847..66e7c6461ecd73368c7dc4823148ad9de2490148 100644 (file)
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -542,6 +542,10 @@ most platforms.  On Fedora and SuSE, it is equal to ``"lib64"`` on 64-bit
 platforms.
 (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in :issue:`1294959`.)
 
+Previously, :attr:`sys.stderr` was block-buffered when non-interactive. Now
+``stderr`` defaults to always being line-buffered.
+(Contributed by Jendrik Seipp in :issue:`13601`.)
+
 
 typing
 ------