Remove an unnecessary extra copy of the bytes hash function.

Remove an unnecessary extra copy of the bytes hash function.

This copy also had a bug in it, it fails to incorporate the length
into the hash by using it as the loop variable so it'll always be -1
by the time it is XORed in.

As such: I'm doing this only in Python 3.3 and not backporting as it
would change the existing hash behavior of datetime objects.