hashlib calling sequence

It now needs bytes so this
 `signature = hashlib.md5("%s%s" % (hashlib.md5("%s%s" % (data, self.private_key)).hexdigest(),  self.private_key)).hexdigest()`
 
should be

` signature = hashlib.md5((hashlib.md5((data+ self.private_key).encode()).hexdigest()+self.private_key).encode()).hexdigest()`