devel/py-build: restore USE_PYTHON=allflavors but limit to 3.8+

On Python 3.7, the importlib-metadata dependency tree is not yet
allflavors-safe. For the time being, the PEP-517 build frontend
role on that Python version is filled by gpep517.

With hat: python

devel/py-build/Makefile

@@ -15,8 +15,8 @@ LICENSE_FILE=	${WRKSRC}/LICENSE
 RUN_DEPENDS=	${PYTHON_PKGNAMEPREFIX}packaging>=19.0:devel/py-packaging@${PY_FLAVOR} \
 		${PYTHON_PKGNAMEPREFIX}pyproject_hooks>=0:devel/py-pyproject_hooks@${PY_FLAVOR}
 
-USES=		python
-USE_PYTHON=	autoplist concurrent pep517
+USES=		python:3.8+
+USE_PYTHON=	allflavors autoplist concurrent pep517
 
 NO_ARCH=	yes