devel/boost-python-libs: mark as not compatible with python 3.10

This limitation is inherited from math/py-numpy and needs to be
rechecked when it supports 3.10

Approved by:	portmgr blanket

devel/boost-python-libs/Makefile

@@ -10,7 +10,7 @@ BUILD_DEPENDS=	bjam:devel/boost-jam \
 		${PYNUMPY}
 LIB_DEPENDS=	libboost_thread.so:devel/boost-libs
 
-USES=		python:3.6+
+USES=		python:3.6-3.9  # upper bound inherited from math/py-numpy
 USE_PYTHON=	flavors
 
 OPTIONS_DEFINE=	DEBUG OPTIMIZED_CFLAGS