If you use pubkeys.txt from https://www.python.org/static/files/pubkeys.txt, then GPG verification gives you no additional security. An attack with write access to www.python.org or access to the private key of www.python.org can easily replace the pubkeys.txt with a key file under his control. You only get additional security if you retrieve the key from a different location *and* verify that the key owned by Łukasz.