If I understand correctlty, _PyUnicode_AsDefaultEncodedString creates and caches a utf8 translation of the string. It returns a borrowed reference to this.
But the stored value (in ->defenc) has a refcount of 2, and will never be released when the unicode object is freed.

The effect is obvious in debug build: the total references count used to increase by 1 for each function call.

The patch is easy:

Index: Objects/unicodeobject.c
=======================================================
--- Objects/unicodeobject.c     (revision 56284)
+++ Objects/unicodeobject.c     (working copy)
@@ -1207,7 +1213,6 @@
                                    PyBytes_Size(b));
     Py_DECREF(b);
     if (!errors) {
-        Py_XINCREF(v);
         ((PyUnicodeObject *)unicode)->defenc = v;
     }
     return v;