Issue #13149: Speed up append-only StringIO objects.
This is very similar to the "lazy strings" idea.
diff --git a/Misc/NEWS b/Misc/NEWS
index 48094e3..2841fb7 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -365,6 +365,8 @@
 Library
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+- Issue #13149: Speed up append-only StringIO objects.
+
 - Issue #13373: multiprocessing.Queue.get() could sometimes block indefinitely
   when called with a timeout.  Patch by Arnaud Ysmal.