gh-99300: Replace Py_INCREF() with Py_NewRef() in _elementtree.c (#99696)

* Replace Py_INCREF() and Py_XINCREF() using a cast with Py_NewRef()
  and Py_XNewRef() in Modules/_elementtree.c.
* Make reference counting more explicit: don't steal implicitly a
  reference on PyList_SET_ITEM(), use Py_NewRef() instead.
* Replace PyModule_AddObject() with PyModule_AddObjectRef().