This is my nearly two year old patch

[ 400998 ] experimental support for extended slicing on lists

somewhat spruced up and better tested than it was when I wrote it.

Includes docs & tests.  The whatsnew section needs expanding, and arrays
should support extended slices -- later.
diff --git a/Misc/NEWS b/Misc/NEWS
index 752082b..9c51c1f 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -6,6 +6,10 @@
 
 Core and builtins
 
+- Most builtin sequences now support "extended slices", i.e. slices
+  with a third "stride" parameter.  For example, "range(10)[1:6:2]"
+  evaluates to [1, 3, 5].
+
 - Cycles going through the __class__ link of a new-style instance are
   now detected by the garbage collector.