This will slow down performance because it will take two passes through the list.

it just removes a few validations, which is theoratically faster, but not much

Yeah, thats what I did. I wasn't sure if it was actually faster though, so I timed it with the time module and it was faster.

This was the initial code:

import string
from codecs import encode as _dont_use_this_

def rot13(message):
    pass

Have you deleted the import line?

Either way, I've edited the Python translation and fixed that and enabled Python 3.6.