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().

Modules/_struct.c

@@ -121,7 +121,7 @@ get_pylong(PyObject *v)
     if (!PyLong_Check(v)) {
         /* Not an integer;  try to use __index__ to convert. */
         if (PyIndex_Check(v)) {
-            v = PyNumber_Index(v);
+            v = _PyNumber_Index(v);
             if (v == NULL)
                 return NULL;
         }