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_httplib.py

@@ -157,7 +157,8 @@ class BasicTest(TestCase):
         sock = FakeSocket(body)
         conn.sock = sock
         conn.request('GET', '/foo', body)
-        self.assertTrue(sock.data.startswith(expected))
+        self.assertTrue(sock.data.startswith(expected), '%r != %r' %
+                (sock.data[:len(expected)], expected))
 
 class OfflineTest(TestCase):
     def test_responses(self):