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

Backport of r66119
diff --git a/Misc/NEWS b/Misc/NEWS
index d2d58a5..0f00535 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 #3537: Fix an assertion failure when an empty but presized dict
   object was stored in the freelist.