Because media-libs/mesa dies in pkg_setup when PYTHON_TARGETS and PYTHON_SINGLE_TARGET are not set appropriately, the ebuild should set REQUIRED_USE appropriately. The python-single-r1 eclass provides a variable with the appropriate expression, so the only change to the ebuild required is to add ${PYTHON_REQUIRED_USE} to REQUIRED_USE. (The reason the eclass doesn't do this itself is so that the ebuild can make that requirement itself conditional on another USE flag, such as "python? ( ${PYTHON_REQUIRED_USE}").
May be you can provide patch?
Fixed in 9.2.5-r1 and 10.0.3
