Fix python 2.4 compatibility

I got overexcited and forgot we have to remain compatible with python
2.4. With this patch we move from sha256 to sha1 for hmac authenticated
serialized messages, and we handle both newer and older python, by
importing the right module for each.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>