Add an index entry for "subclassing immutable types".

Doc/reference/datamodel.rst

@@ -1162,9 +1162,10 @@ of this is the :class:`NodeList` interface in the W3C's Document Object Model.)
 Basic customization
 -------------------
 
-
 .. method:: object.__new__(cls[, ...])
 
+   .. index:: pair: subclassing; immutable types
+
    Called to create a new instance of class *cls*.  :meth:`__new__` is a static
    method (special-cased so you need not declare it as such) that takes the class
    of which an instance was requested as its first argument.  The remaining