editors/setzer: New port, version 0.3.4

Write LaTeX documents with an easy to use yet full-featured editor.

Rapidly evolving editor, based on Python 3 and Gtk3.

Homepage[1] and repository[2]:
[1] https://www.cvfosammmm.org/setzer/
[2] https://github.com/cvfosammmm/Setzer

PR: 247063
Submitted by: Greg V <greg@unrelenting.technology> (maintainer)
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D26834