I just voted to close this question, as it was really not about Mathematica but about Mathematica equivalents in other languages like Python. One of the sub-options to "Close as off-topic" is "Belongs on another stackexchange site". The only two choices for other sites, though, are meta.mathematica.se or math.se. Shouldn't stackoverflow be an option here? Can we make it so?
Just flag as you have done. Moderators can migrate to other sites if needed, and I have done so in this instance.
As Verbeia has already mentioned, to migrate something to SO, just flag for moderator attention. Regarding the second part of your question — "Shouldn't stackoverflow be an option here? Can we make it so?", adding a migration option is done only if there's sufficient volume to warrant it (even if a site seems like a perfect migration target).
You can see the migration stats for the past 90 days in your 10k tools page. Here's the list for those who can't see it:
You can see that in past 3 months, we've migrated only 2 questions (including the question you mentioned above), of which one was rejected. In comparison, we've migrated 11/month with only a 9% rejection rate to Mathematics, which is why we have that line open.
Along the same lines, Mathematics used to keep sending us crap (even now, 2/3 rejected), which is why their request to open a direct migration path to us was declined.