layman-2.1.0-r1 relies on asciidoc, which is unfortunately only available for Python2 and prevents a Python3-only installation if layman is installed. I've tried to install layman's ebuild without asciidoc, everything's installed fine, including the man page (which is why asciidoc was supposed to be needed, AFAIK). Is asciidoc really needed, or is that just an old line needed for <2.1.0? Oh, and thanks a lot for the Python3 support ;).
Fixed with the addition of layman-2.1.0-r2, thanks for pointing out this issue :) The new ebuild will be on the portage tree within a few hours.