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().
diff --git a/Objects/floatobject.c b/Objects/floatobject.c
index cc0ae8c..868b729 100644
--- a/Objects/floatobject.c
+++ b/Objects/floatobject.c
@@ -250,7 +250,7 @@
     nb = Py_TYPE(op)->tp_as_number;
     if (nb == NULL || nb->nb_float == NULL) {
         if (nb && nb->nb_index) {
-            PyObject *res = PyNumber_Index(op);
+            PyObject *res = _PyNumber_Index(op);
             if (!res) {
                 return -1;
             }