import: Support importing .7z archives

[Freso]

Use case

I'm trying to use beets to import music currently bundled in .7z archives.

Solution

Should work similar to how importing of .zip and .rar etc. works.

Alternatives

There is currently a beets feature supporting import for archives, however, it doesn’t (currently) support .7z archives. There seem to be some Python libraries available for handling .7z/LZMA, but not sure if any of them are directly usable by beets (ie., they don’t seem to share a similar interface to the current archive extractors from a cursory glance).

[wisp3rwind]

Sounds like a nice addition! If you've already done some research on 7z libraries, maybe you could link them here? That might be useful for anyone who's interested in implementing it. I don't think the current unzipping stage is set in stone, see also the discussion in #3538.

[Freso]

I have not done much more research than going to https://pypi.org/search/?q=lzma and https://pypi.org/search/?q=7z and looking at some of the modules, trying to see if they had a similar API to what is currently used by beets. Don’t remember which specific ones I looked at.