Fix SF bug #667147, Segmentation fault printing str subclass

Fix infinite recursion which occurred when printing an object
whose __str__() returned self.

Will backport
diff --git a/Misc/NEWS b/Misc/NEWS
index 1fa1c8c..5f415d4 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -18,6 +18,9 @@
   Passing None is semantically identical to calling sort() with no
   arguments.
 
+- Fixed crash when printing a subclass of str and __str__ returned self.
+  See SF bug #667147.
+
 Extension modules
 -----------------