ci.yml: Upgrade GitHub Actions

[cclauss]

• https://github.com/actions/checkout/releases
• https://github.com/actions/setup-python/releases

Let's get the GitHub Actions passing again.

Description

Please include a summary of the changes and which issue this relates to (if applicable).

Fixes # (issue)

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] I have added an appropriate CHANGELOG entry