The package specifies: python? ( [...] || ( deprecated ( gtk3 introspection ) ) ) which is horribly confusing, and most likely invalid. Judging by the other dependencies and ebuild functions, USE=python+gtk seems to explicitly require introspection enabled, and I honestly doubt that anymore using this combination of flags would prefer enabling USE=deprecated over USE=introspection. Therefore, I believe this constraint should be replaced with: python? ( [...] || ( deprecated gtk3 ) gtk3? ( introspection ) )"
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=642f19aeb12bae067896af2116c79f0327ad037a