Improve extended slicing support in builtin types and classes. Specifically:

- Specialcase extended slices that amount to a shallow copy the same way as
   is done for simple slices, in the tuple, string and unicode case.

 - Specialcase step-1 extended slices to optimize the common case for all
   involved types.

 - For lists, allow extended slice assignment of differing lengths as long
   as the step is 1. (Previously, 'l[:2:1] = []' failed even though
   'l[:2] = []' and 'l[:2:None] = []' do not.)

 - Implement extended slicing for buffer, array, structseq, mmap and
   UserString.UserString.

 - Implement slice-object support (but not non-step-1 slice assignment) for
   UserString.MutableString.

 - Add tests for all new functionality.

Lib/test/test_structseq.py

@@ -97,6 +97,18 @@ def test_reduce(self):
         t = time.gmtime()
         x = t.__reduce__()
 
+    def test_extended_getslice(self):
+        # Test extended slicing by comparing with list slicing.
+        t = time.gmtime()
+        L = list(t)
+        indices = (0, None, 1, 3, 19, 300, -1, -2, -31, -300)
+        for start in indices:
+            for stop in indices:
+                # Skip step 0 (invalid)
+                for step in indices[1:]:
+                    self.assertEqual(list(t[start:stop:step]),
+                                     L[start:stop:step])
+
 def test_main():
     test_support.run_unittest(StructSeqTest)