Issue #2115: __slot__ attributes setting was 10x slower.
Also correct a possible crash using ABCs.

This change is exactly the same as an optimisation
done 5 years ago, but on slot *access*:
http://svn.python.org/view?view=rev&rev=28297
diff --git a/Misc/NEWS b/Misc/NEWS
index fdc81c1..74466d6 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,10 @@
 Core and builtins
 -----------------
 
+- Issue #2115: Important speedup in setting __slot__ attributes.  Also 
+  prevent a possible crash: an Abstract Base Class would try to access a slot 
+  on a registered virtual subclass.
+
 - Fixed repr() and str() of complex numbers with infinity or nan as real or
   imaginary part.