Use SourceForge auto mirror capability

[raygard]

Use SourceForge auto mirror capability.

Fixes #6343 .

Changes proposed in this pull request:

• Use SourceForge auto mirror capability

[nulano]

#6343 (comment):

@nulano this looks to be above my comfort level with GitHub etc. I hope someone else can help me with this. I think the PR is fine but would love some help before it goes out to the world.

I've made the changes in a new branch: https://github.com/raygard/Pillow/compare/winbuild_use_sourceforge_auto_mirror...nulano:patch-1

The CI runs are here:
GitHub Actions: https://github.com/nulano/Pillow/actions/runs/2425275727
AppVeyor: https://ci.appveyor.com/project/nulano/pillow/builds/43732459

The runs are successfully downloading the dependencies:

Fetching https://sourceforge.net/projects/libjpeg-turbo/files/2.1.3/libjpeg-turbo-2.1.3.tar.gz/download (attempt 1)...
Extracting libjpeg-turbo-2.1.3.tar.gz

[hugovk]

3.11-dev failed yesterday because the new 3.11 beta 2 didn't play nicely with pytest, but they fixed it with an expedited 3.11 beta 3.

I've restarted the failing 3.11-dev jobs and they now pass 👍

Thanks all!