Merging the py3k-pep3137 branch back into the py3k branch.
No detailed change log; just check out the change log for the py3k-pep3137
branch.  The most obvious changes:

  - str8 renamed to bytes (PyString at the C level);
  - bytes renamed to buffer (PyBytes at the C level);
  - PyString and PyUnicode are no longer compatible.

I.e. we now have an immutable bytes type and a mutable bytes type.

The behavior of PyString was modified quite a bit, to make it more
bytes-like.  Some changes are still on the to-do list.
diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
index 9670be0..4f84328 100644
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -580,8 +580,7 @@
         self.assertEqual(hash(1), hash(1))
         self.assertEqual(hash(1), hash(1.0))
         hash('spam')
-        self.assertEqual(hash('spam'), hash(str8(b'spam'))) # remove str8()
-                                                    # when b"" is immutable
+        self.assertEqual(hash('spam'), hash(b'spam'))
         hash((0,1,2,3))
         def f(): pass
         self.assertRaises(TypeError, hash, [])