It's not particularly unusual in Python, because it's normal that important functionality is implemented in other languages by a diverse set of third parties.

That said, this isn't a security vulnerability, it's just a bug. To meet a reasonable threshold for being a security issue, you need to show a real system that has an issue caused by this, and then the vulnerability is in that system, rather than in Python.

I'll grudgingly allow that a buffer overflow or an SQL injection possibility - in a library advertised as safe against that kind of bug - is a security issue, because there's so much history of turning those into real exploits. But a choice of library or language that makes those bugs easier to write - the idna library, or C or PHP say, is not itself a security issue.