Because dev-libs/libnatspec-0.2.6-r1 inherits python-single-r1 and does not implement its own pkg_setup(), the eclass assumes that python support is always required. As it is optional in this case, the python-single-r1_pkg_setup function should itself be called conditionally, like so: pkg_setup() { use python && python-single-r1_pkg_setup }
Same problem here, please fix.
+ 26 Jul 2015; Michał Górny <mgorny@gentoo.org> libnatspec-0.2.6-r1.ebuild: + Add proper pkg_setup() to fix USE=-python, bug #546212. + https://github.com/gentoo/gentoo-portage-rsync-mirror/pull/185 by a17r.