several dev-python/pypy-bin and dev-python/pypy3-bin ebuilds have nonexistent, slotted versions of dev-libs/libffi as their dependencies:

dev-libs/libffi:0/0

/var/git/meta-repo/kits/python-modules-kit/dev-python/pypy3-bin/pypy3-bin-6.0.0.ebuild
/var/git/meta-repo/kits/python-modules-kit/dev-python/pypy3-bin/pypy3-bin-5.10.1.ebuild
/var/git/meta-repo/kits/python-modules-kit/dev-python/pypy-bin/pypy-bin-6.0.0.ebuild
/var/git/meta-repo/kits/python-modules-kit/dev-python/pypy-bin/pypy-bin-5.10.0.ebuild

dev-libs/libffi:0/7

/var/git/meta-repo/kits/python-modules-kit/dev-python/pypy3-bin/pypy3-bin-6.0.0-r100.ebuild
/var/git/meta-repo/kits/python-modules-kit/dev-python/pypy-bin/pypy-bin-6.0.0-r100.ebuild