math/py-gmpy2: limit supported python versions

src/gmpy_mpfr.c:756:20: error: use of undeclared identifier '_PyHASH_NAN'
            return _PyHASH_NAN;

Approved by:	portmgr blanket

math/py-gmpy2/Makefile

@@ -16,7 +16,7 @@ LIB_DEPENDS=	libgmp.so:math/gmp \
 		libmpc.so:math/mpc \
 		libmpfr.so:math/mpfr
 
-USES=		python:3.6+ zip
+USES=		python:3.6-3.9 zip
 USE_PYTHON=	distutils autoplist
 
 CONFLICTS=	${PYTHON_PKGNAMEPREFIX}gmpy2-devel-2.1*