[Python] QHelp references old-ish python 2 documentation

I was looking at the [py/modification-of-default-value](https://codeql.github.com/codeql-query-help/python/py-modification-of-default-value/) query and noticed that it still refers to python 2 documentation.
A quick search reveals that there are more such references:
https://github.com/search?q=repo%3Agithub%2Fcodeql+%22python.org%2F2%22+language%3AXML&type=code&l=XML

While python 2.7 is still officially supported by CodeQL I think it would make sense to update those references to python 3.
But I of course also understand that this is probably a low priority issue :)

Start with the py/modification-of-default-value query-help page and the provided repository search for python.org/2 references in XML. Review each matching reference and update applicable Python 2 documentation links to Python 3. Done means the outdated references identified by the search have been updated without changing references that still support Python 2.

Rédigé par le modèle d'indexation à partir du texte de l'issue.