Fix incorrect `Py_XDECREF/Py_DECREF` call on stolen reference after `PyList_SetItem`

Hi, I found that this issue has been acknowledged and fixed upstream. I think this project should also apply the fix. Could you merge this PR or sync the latest libxml2?

https://github.com/GNOME/libxml2/commit/046931e64e9e081e82c341f6455d070716fb8cf2