Fix str.rpartition(sep) when sep is not found in str.
Partially from SF patch #1551339, but also taken from head.
diff --git a/Misc/NEWS b/Misc/NEWS
index 7883c18..56f13fa 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -27,6 +27,9 @@
 
 - Patch #1546288: fix seg fault in dict_equal due to ref counting bug.
 
+- The return tuple from str.rpartition(sep) is (tail, sep, head) where
+  head is the original string if sep was not found.
+
 
 Library
 -------