bpo-40792: Make the result of PyNumber_Index() always having exact type int. (GH-20443)

Previously, the result could have been an instance of a subclass of int.

Also revert bpo-26202 and make attributes start, stop and step of the range
object having exact type int.

Add private function _PyNumber_Index() which preserves the old behavior
of PyNumber_Index() for performance to use it in the conversion functions
like PyLong_AsLong().

Objects/typeobject.c

@@ -6334,7 +6334,7 @@ slot_sq_length(PyObject *self)
     if (res == NULL)
         return -1;
 
-    Py_SETREF(res, PyNumber_Index(res));
+    Py_SETREF(res, _PyNumber_Index(res));
     if (res == NULL)
         return -1;