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.

Lib/test/test_marshal.py

@@ -39,7 +39,7 @@ class IntTestCase(unittest.TestCase, HelperMixin):
         # we're running the test on a 32-bit box, of course.
 
         def to_little_endian_string(value, nbytes):
-            b = bytes()
+            b = buffer()
             for i in range(nbytes):
                 b.append(value & 0xff)
                 value >>= 8