Author chiming in.
Some subtitle providers send zips (such as OpenSubtitles) instead of the actual subtitle file.
In some cases (like legendastv), they might send a RAR file. Python handles ZipFiles internally but unraring implies the unrar dependency.
LegendasTV has only been added to the 2.0 branch and release candidates of the new Sub-Zero, and chopeta is a beta tester. That's why he found that out.