Add __del__ callbacks.  They are too useful to leave out.

XXX Remaining problems:

- The GC module doesn't know about these; I think it has its reasons
  to disallow calling __del__, but for now, __del__ on new-style
  objects is called when the GC module discards an object, for better
  or for worse.

- The code to call a __del__ handler is really ridiculously
  complicated, due to all the different debug #ifdefs.  I've copied
  this from the similar code in classobject.c, so I'm pretty sure I
  did it right, but it's not pretty. :-(

- No tests yet.
diff --git a/Misc/NEWS b/Misc/NEWS
index 2a29558..55e916b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -4,6 +4,9 @@
 
 Type/class unification and new-style classes
 
+- New-style classes can now have a __del__ method, which is called
+  when the instance is deleted (just like for classic classes).
+
 - Assignment to object.__dict__ is now possible, for objects that are
   instances of new-style classes that have a __dict__ (unless the base
   class forbids it).