As far as a license on the CML schema itself goes, I have found none. I also found no license in the repo linked to in the question. It's not clear that this is in fact a maintained repo because the last changes all seem to have been made 2-3 years ago.
Also, I would encourage anyone applying the Artistic License to their own work or using software under its terms to read this from Lawrence Rosin, the lawyer who wrote the book on the subject of open source licenses. Then run it by your own lawyer.
The Artistic License is one such amateur license. It is a
license that a lawyer would have difﬁculty explaining and that a
judge would probably not be able to understand. ... I know what those authors
were trying to say, and I support their goals of artistic attribution and integrity, but I believe they made a legal mess of it.
There are better licenses to choose from that accomplish the same goals. Read Rosin's book and see for yourself. Getting it right from the start will prevent all kinds of headache - tracking down and getting the consent for a license change from each contributor to a project three years after the fact may not even be possible.