code: def sign(self, message):
  		
  		return hmac.HMAC(self.key, message, digestmod=hashlib.md5).digest()
docs: >>> authlen = OmapiHMACMD5Authenticator.authlen
  		>>> len(OmapiHMACMD5Authenticator(b"foo", 16*b"x").sign(b"baz")) == authlen
  		True
  
  		@type message: bytes
  		@rtype: bytes
  		@returns: a signature of length self.authlen