New port: sysutils/py-pyznap: ZFS snapshot tool written in Python

pyznap is a ZFS snapshot management tool. It automatically takes and deletes
snapshots and can send them to different remote and local backup locations.

For example, it is possible to specify a policy for a given filesystem in the
pyznap.conf file and then use cron to let it run once per quarter-hour.

WWW: https://github.com/yboetz/pyznap

PR: 232161
Submitted by: Dries Michiels <driesm.michiels@gmail.com> (previous version)
Reviewed by: krion
Approved by: krion (mentor)
Differential Revision: https://reviews.freebsd.org/D17520