[Beltalowda]

That older Python 2 answer can be edited/amended to either include the "Python 3 way" (if it's simple enough), or to add a notice that there's a better/newer way of doing this in Python 3, possibly with a link to another answer.

I don't think they way Stack Overflow deals with this is perfect, but there are tools to deal with this.

[Someone]

Can users other than whoever wrote the to-be-updated answer do that?

I think it requires action by either the poser of the question (if they can change the ‘winning’ answer months or years after picking it) or the user writing the winning answer.

I think it’s unlikely they’ll do that.

[Beltalowda]

Anyone can edit anything; you don't even need an account. Edits from users with <2,000 reputation and anonymous edits need to be approved, and it's not a wiki where you can radically depart from the original author's intent, but many answer have improvements, additions, etc.