Configure checks for python-2.7 which is not included in stage3 tarball so the dep must be included in ebuild to ensure package can be built.
I had forgotten to add PYTHON_DEPS to RDEPEND/DEPEND when updating the ebuild to python-r1.eclass. Thanks for noticing! + 03 Jan 2013; Alexandre Rostovtsev <tetromino@gentoo.org> + gobject-introspection-1.34.2-r1.ebuild: + Forgot to add PYTHON_DEPS to dependencies (bug #449824, thanks to Jory A. + Pratt).