devel/py-blist: limit supported python versions

blist/_blist.c:6592:27: error: expression is not assignable
        Py_REFCNT(&saved) = 1;

devel/py-blist/Makefile

@@ -13,7 +13,7 @@ COMMENT=	Drop-in list replacement with better performance for large lists
 LICENSE=	BSD3CLAUSE
 LICENSE_FILE=	${WRKSRC}/LICENSE
 
-USES=		python:3.6+
+USES=		python:3.6-3.9
 USE_PYTHON=	autoplist distutils
 
 do-test: