Issue #3751: str.rpartition would perform a left-partition when called with
a unicode argument.

will backport.
diff --git a/Misc/NEWS b/Misc/NEWS
index d8145c3..6f012ab 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@
 Core and Builtins
 -----------------
 
+- Issue #3751: str.rpartition would perform a left-partition when called with
+  a unicode argument.
+
 - Issue #3683: Fix compilation when --without-threads is given.
 
 - Issue #3668: Fix a memory leak with the "s*" argument parser in