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_asyncore.py

@@ -70,7 +70,6 @@ def capture_server(evt, buf):
             r, w, e = select.select([conn], [], [])
             if r:
                 data = conn.recv(10)
-                assert isinstance(data, bytes)
                 # keep everything except for the newline terminator
                 buf.write(data.replace(b'\n', b''))
                 if b'\n' in data: