GHA: Replace 3.9 beta with 3.9 final

[hugovk]

Summary of changes

The CI is failing because it's failing to set up 3.9-beta via deadsnakes.

 /usr/bin/python3.9 -mvenv /home/runner/venv-3.9 Error: Command '['/home/runner/venv-3.9/bin/python3.9', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 1. Error: The process '/usr/bin/python3.9' failed with exit code 1 

• https://github.com/pypa/setuptools/runs/1239771433?check_suite_focus=true

Python 3.9.0 final is now available on GitHub Actions as 3.9.

Let's add that, and we can remove the betas:

• 3.9.0-beta.4 - 3.9.0 from GHA
• 3.9-beta from deadsnakes

I left in the 3.8-dev and 3.9-dev dev versions from deadsnakes. Still useful to keep them both?

I also left in the logic for checking -beta, will be useful for when 3.10 reaches beta (or to adapt for alpha). Let me know if you'd prefer it cleaned up.

---

Also we can see from the above that the single failing job caused all the other jobs to be auto-cancelled:

• https://github.com/pypa/setuptools/runs/1239771433?check_suite_focus=true

Instead, we can use fail-fast: false which allows the other jobs to run. For example:

• https://github.com/hugovk/setuptools/actions/runs/301588612

Pull Request Checklist

• Changes have tests [changes sort of are tests]
• News fragment added in changelog.d. See documentation for details

[webknjaz]

I think it's worth adding a misc change note here

[webknjaz]

It's funny that I was just about to submit exactly this change right before seeing your PR. So I ended up sending only the metadata update: #2421.

[webknjaz]

By the way, the original problem was caused by the upstream bug:
deadsnakes/issues#138 (comment)

[webknjaz]

@jaraco this PR is clear to be merged w/o any extra reviews. We need to get this done ASAP before anything else in order to fix the CI that makes all of the PR submissions look broken while it's broken because it points to a deadsnakes release that is no longer there.