According to the Guideline "please use the original title, unless it is misleading or linkbait; don't editorialize" the Title should be "What’s New In Python 3.9"+
It's a strange HN-ism that puzzles me - "OK, what's so important about Foo 3.14?" There is a rule that is (barely IMO) on point though in the posting guidelines you linked to:
> If the title begins with a number or number + gratuitous adjective, we'd appreciate it if you'd crop it. E.g. translate "10 Ways To Do X" to "How To Do X," and "14 Amazing Ys" to "Ys." Exception: when the number is meaningful, e.g. "The 5 Platonic Solids."