Because media-libs/mlt-0.9.8 only optionally uses python, it should have the following: REQUIRED_USE="python? ( ${PYTHON_REQUIRED_USE} )" pkg_setup() { use python && python-single-r1_pkg_setup } Otherwise, the package will have errors on what should be correct sets of USE flags.
Thanks, this is fixed in git by Andreas. https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=4f6d52106be38419f309add0dc4df7e3ce3bd859