Catalyst currently has a hard dependency on app-text/asciidoc. Unfortunately, the current asciidoc packages in Gentoo only work with Pythono-2.7, which is going to be EOL'ed in 2020, thus you have to set PYTHON_SINGLE_TARGET="python2_7".
This becomes a problem if you have to install dev-util/glib-utils (which usually happens if you ever have to install dev-libs/glib), which only supports Python-3.x now, as far as I can tell. In order to avoid a block loop, you have to set PYTHON_SINGLE_TARGET to a supported python3_X value. This will of course then break asciidoc.
Bug #661662 is open to request addition of a Python-3.x-compatible version of asciidoc, via a new asciidoc-py3 package, but not much of an ETA on when that will be complete.
It looks like asciidoc has no use after catalyst is installed, so I think the easiest way to resolve this for now is to make asciidoc an optional dependency in catalyst behind a USE flag, as well as the corresponding build logic in catalyst's compile phase.
asciidoc with python3 support is stable everywhere. There's also a Ruby implementation (dev-ruby/asciidoctor). I don't know that it makes sense to make asciidoc optional at this point.