net/py-avahi can't be installed, because it depends on devel/py-dbus which is python2 only. Even after installing that one, the file avahi is looking for can't be found.
Either mark the port python:2 only, or implement a mechanism to detect the installed python default.
Also note that it would be a good idea to deprecate pkg-plist in favor of autoplist, if Python3 support is what is aimed at.
A commit references this bug:
Date: Wed May 17 10:35:04 UTC 2017
New revision: 441076
py-avahi is python 2 only.
Submitted by: xmj@
Committed the python 2 restriction, thanks!