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/Lib/test/test_descr.py b/Lib/test/test_descr.py
index 755a967..3c607f7 100644
--- a/Lib/test/test_descr.py
+++ b/Lib/test/test_descr.py
@@ -1182,6 +1182,24 @@
         a.foo = 42
         self.assertEqual(a.__dict__, {"foo": 42})
 
+    def test_slots_descriptor(self):
+        # Issue2115: slot descriptors did not correctly check
+        # the type of the given object
+        import abc
+        class MyABC:
+            __metaclass__ = abc.ABCMeta
+            __slots__ = "a"
+
+        class Unrelated(object):
+            pass
+        MyABC.register(Unrelated)
+
+        u = Unrelated()
+        self.assert_(isinstance(u, MyABC))
+
+        # This used to crash
+        self.assertRaises(TypeError, MyABC.a.__set__, u, 3)
+
     def test_dynamics(self):
         # Testing class attribute propagation...
         class D(object):