bpo-36048: Use __index__() instead of __int__() for implicit conversion if available. (GH-11952)

Deprecate using the __int__() method in implicit conversions of Python
numbers to C integers.

Doc/c-api/number.rst

@@ -11,6 +11,9 @@ Number Protocol
    Returns ``1`` if the object *o* provides numeric protocols, and false otherwise.
    This function always succeeds.
 
+   .. versionchanged:: 3.8
+      Returns ``1`` if *o* is an index integer.
+
 
 .. c:function:: PyObject* PyNumber_Add(PyObject *o1, PyObject *o2)