1) This port blocks the change of the default version of Python from 3.8
to 3.9 (See PR 261703);
2) To fix it with Python-3.9, we have to upgrade it (patch attached in
PR 262797);
3) But this upgrade requires to install a new port, devel/stanc3 (see PR
262795 and PR 262796);
4) And finally to add devel/stanc3 we need an infrastructure of OCaml
ports relatively recent (see PR 262781);
5) Unfortunately it does not seem realistic to expect this modernization
of OCaml ports before a rather long time.
Meanwhile, mark it BROKEN with Python-3.9.
PR: 262797
Approved by: Gilbert Morgan (maintainer)
6003b7e9ad