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_multibytecodec_support.py b/Lib/test/test_multibytecodec_support.py
index e51be04..957b9fc 100644
--- a/Lib/test/test_multibytecodec_support.py
+++ b/Lib/test/test_multibytecodec_support.py
@@ -52,6 +52,10 @@
                 func = self.encode
             if expected:
                 result = func(source, scheme)[0]
+                if func is self.decode:
+                    self.assert_(type(result) is str, type(result))
+                else:
+                    self.assert_(type(result) is bytes, type(result))
                 self.assertEqual(result, expected)
             else:
                 self.assertRaises(UnicodeError, func, source, scheme)