Test for backward compatibility against multiple other projects

[amyreese]

#205 adds a simple test script that validates sorting results against previous versions of µsort. However, it only tests this by sorting the µsort codebase itself. We should also include coverage of more and larger codebases, like black, libcst, torch-vision, etc. Maybe prefer to check out these projects from pinned revs, rather than latest versions, for consistency of results.

[thatch]

Sounds good. I think we should also have an artificial repo containing as much interesting/weird constructs as we can, such as the

Import a as c
Import b as c

That were unlikely to find in real code