Added note of unittest.py changes that fixed bug 451309
diff --git a/Misc/NEWS b/Misc/NEWS
index 52b2a1e..ceb1c06 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -108,6 +108,10 @@
 - In restricted execution mode (using the rexec module), unmarshalling
   code objects is no longer allowed.  This plugs a security hole.
 
+- unittest.TestResult instances no longer store references to tracebacks
+  generated by test failures. This prevents unexpected dangling references
+  to objects that should be garbage collected between tests.
+
 Tools
 
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