Seems like an excellent suggestion, and a natural extension of the error message improvements we’ve been getting over the last few Python versions!
One thing to note is that there are only 4 custom SyntaxErrors from other languages, but your AttributeError list already has 11 items. This isn’t a bad thing, but I could see this getting out of hand quickly if there isn’t at least some simple guidelines around adding new items to the list (do we need a new way to spell ‘append’)?
So I’d say that option 1 is the best way to go:
If at some point in the future we find that extensibility is actually useful here, it can be added later.