Raymond, could you please look at the change in Objects/setobject.c? It makes the code slightly faster (by avoiding few function calls: PyObject_Size(), PySequence_Size()/PyMapping_Size() and sq_length/mp_length), but the main purpose is making clear that no error check is needed after calling PyObject_Size(). I'm not going to include this change in backports.

Xiang suggests larger change that allows to gets rid of one PyDict_CheckExact(). What do you prefer?