As of r439592 (2017-04-27), python 3.6 is the default python3 on FreeBSD. I believe NO_PACKAGE and the pre-everything hook can both be removed now. It works for me with an empty make.conf.
A commit references this bug: Author: araujo Date: Tue Jun 20 02:44:46 UTC 2017 New revision: 443925 URL: https://svnweb.freebsd.org/changeset/ports/443925 Log: - As of r439592 (2017-04-27), python 3.6 is the default python3 on FreeBSD so we can remove NO_PACKAGE. PR: ports/220098 Submitted by: Randy Westlund <rwestlun@gmail.com> Changes: head/sysutils/py3-iocage/Makefile
The pre-everything hook that tells the user to set DEFAULT_VERSIONS can also be removed.