In Python 2 there was no 'binary data' type - everything was a string. Now we have string, str, bytearray, array, list of ints. If hexlify is not accepting anything except bytes, it is better be explicit.

When porting code from Python 2 the argument was passed as a string with escapes inside, so it took some time to figure out why it didn't work in Py3k (actually it took a lot of time, because the research path turned wrong way at this point).