multimedia/mythtv: update to 31.0
- Upgrade multimedia/mythtv and multimedia/mythtv-frontend from 30 to 31.0
- Bumps python to 3.5+
- Uses ffmpeg 4.3.1 so CVE-2016-10191 no longer applies
- Configuration options changed to reflect update
Release notes: https://www.mythtv.org/wiki/Release_Notes_-_31
audio/mythplugin-mythmusic also bumped as a reverse dep.
Pass maintainership of multimedia/mythtv* to submitter.
Submitted by: Alan Hicks <ahicks p-o co uk>