Fix getting the milter running with python3
Turns out the file descriptor we're recieving is in bytes while
email_from_file expects a unicode stream in python3.
So if we run the code using python3, use email_from_binary_file
and it's is python2, keep using the email_from_file.
We also need to encode all strings to bytes before it's passed on to
hashlib.
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>