Add Py_RETURN_NOTIMPLEMENTED macro. Fixes #12724.

Author: briancurtin

Include/object.h

@@ -792,6 +792,10 @@ not implemented for a given type combination.
 PyAPI_DATA(PyObject) _Py_NotImplementedStruct; /* Don't use this directly */
 #define Py_NotImplemented (&_Py_NotImplementedStruct)
 
+/* Macro for returning Py_NotImplemented from a function */
+#define Py_RETURN_NOTIMPLEMENTED \
+    return Py_INCREF(Py_NotImplemented), Py_NotImplemented
+
 /* Rich comparison opcodes */
 #define Py_LT 0
 #define Py_LE 1