Replace SVN with Git for installing extra test images

[hugovk]

GitHub is removing support for Subversion on January 8, 2024.

• https://github.blog/2023-01-20-sunsetting-subversion-support/

Why do we care?

Well, we're using svn in our CI to extract only the test_images directory from https://github.com/python-pillow/pillow-depends.

https://github.com/python-pillow/pillow-depends is a big repo full of binary files.

For our purposes, I think we're better off moving the test images into their own repo, so we can instead simply download the new repo as a zipball.

I've made a new repo at https://github.com/hugovk/test-images, which is just the relevant directory from https://github.com/python-pillow/pillow-depends, and preserves the Git history (following these instructions).

Let's move that repo into https://github.com/python-pillow. Because I don't have permission to create repos in the org, we need two steps:

@aclark4life I've sent a request to transfer the repo to you. Once you accept, please could you transfer it to https://github.com/python-pillow, and check the usual suspects have admin rights?

• Transfer docs here: https://docs.github.com/en/repositories/creating-and-managing-repositories/transferring-a-repository

Once it's transferred, I'll update the paths in this PR.

Checklist

• Transfer repo https://github.com/hugovk/test-images to https://github.com/python-pillow org
• Update paths in this PR (the TODOs)
• Merge this PR
• Remove test_images directory from https://github.com/python-pillow/pillow-depends: Remove test images pillow-depends#41

[radarhere]

I've created hugovk#93 with some further suggestions.