Switch from using unrar to 7z

[tonyg]

• PR is based on the DEVELOP branch
• Don't send big changes all at once. Split up big PRs into multiple smaller PRs that are easier to manage and review
• Read the contribution guide

This little patch experimentally replaces use of unrar (problematic in e.g. #10777) with 7z, which is actually pretty close to drop-in compatible, it seems.

[medariox]

Hey! Thanks for the PR. I don't understand what PySocks is required for. Could you clarify?

[tonyg]

Oops. Sorry, that commit snuck in there. PySocks is required to do anything with a socks proxy at all (AFAICT). I needed the unrar-to-7z thing just to get it to run at all in docker, and then the PySocks thing was notionally separate (though I don't think I understand how socks could ever have worked if PySocks wasn't present!). From the contribution guidelines, I imagine you'd prefer me to redo the branch keeping the two changes separate; let me know if I need to do that or if you're happy to just take it as is (which would be easier for me at least!). Thanks for the software!