generate py3k warnings on __getslice__, __delslice__, and __setslice__

Reviewer: Brett Cannon
diff --git a/Misc/NEWS b/Misc/NEWS
index 9c07eee..ff754df 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -26,6 +26,9 @@
 - Silenced another compiler warning about a used but not defined
   function 'stringlib_contains_obj'.
 
+- Added warnings on the use of ``__getslice__``, ``__setslice__``, or
+  ``__delslice__``.
+
 Library
 -------