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> Would you mind to explain how it's an issue to modify PyObject* temporarily during a function call?
It's not a problem to modify the PyObject* during a function call. However, many functions don't need to modify the object, but are still taking non-const PyObject* arguments.
For example if I have this code:
 if (Py_TYPE(deque) == &deque_type) {
That doesn't modify deque to check the type, but because Py_TYPE casts away the constness, deque can't be a const object.
However, with the new Py_IS_TYPE function:
 if (Py_IS_TYPE(deque, &deque_type)) {
and these two changes:
-static inline int _Py_IS_TYPE(PyObject *ob, PyTypeObject *type) {
+static inline int _Py_IS_TYPE(const PyObject *ob, const PyTypeObject *type) {
 return ob->ob_type == type;
 }
-#define Py_IS_TYPE(ob, type) _Py_IS_TYPE(_PyObject_CAST(ob), type)
+#define Py_IS_TYPE(ob, type) _Py_IS_TYPE(((const PyObject*)(ob)), type)
the deque variable can be const.
Another example of a common pattern that I believe could benefit from this is Py_TYPE(ob)->tp_name. That could be turned into Py_TYPE_NAME(ob) and that would allow the ob to be a const pointer. 
If we can keep functions that don't modify the object to accept const PyObject* it will help make things safer in the long run.