math/py-pystan: mark BROKEN with Python-3.9

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)

math/py-pystan/Makefile

@@ -14,6 +14,13 @@ LICENSE_FILE=	${WRKSRC}/LICENSE
 USES=		compiler:c++14-lang python:3.6+ shebangfix
 USE_PYTHON=	autoplist distutils
 
+.include <bsd.port.pre.mk>
+
+.if ${PYTHON_REL} >= 30900
+# See PR 262797 - to be upgraded after the modernization of OCaml ports
+BROKEN=		several build errors
+.endif
+
 BUILD_DEPENDS+=	${PYTHON_PKGNAMEPREFIX}cython>0:lang/cython@${PY_FLAVOR}
 BUILD_DEPENDS+=	${PYTHON_PKGNAMEPREFIX}numpy>0:math/py-numpy@${PY_FLAVOR}
 
@@ -27,4 +34,4 @@ SHEBANG_FILES=	${WRKSRC}/pystan/stan/lib/stan_math/lib/upgrade-sundials.sh \
 NO_ARCH=	yes
 NO_ARCH_IGNORE=	_api.so _chains.so _misc.so
 
-.include <bsd.port.mk>
+.include <bsd.port.post.mk>