use explicit setup-python versions over implicit expose-

[cburroughs]

The project is currently using a mix of the GitHub supplied setup-python action and pantsbuild/actions/expose-pythons. On GitHub managed runners they behave similarly:

• setup-python will install a version of Python if missing, or add it to the PATH if already present at the expected location (managed runners historically have multiple Pythons baked into the image)
• expose-pythons will add all Python's at the expected GitHub location to the PATH. (Which ones is up to GitHub.)

So today the invocation of setup-python followed by expose-pythons is redundant.

Consolidating on setup-python let's us be explicit about expected versions (more like the ARM image) and reduces the number of custom actions the project manages while still making multiple Python versions available.

NOTE: The awkward double newlines in the final yaml are a pre-existing issue
https://stackoverflow.com/questions/45004464/yaml-dump-adding-unwanted-newlines-in-multiline-strings

See #21552 for some history regarding the setup- vs expose- actions.

[huonw]

Seems very sensible to be explicit about which Python versions are desirable!

[huonw]

Marked for cherrypicking to unblock #21663 and #21664

[WorkerPants]

I tried to automatically cherry-pick this change back to each relevant milestone, so that it is available in those older releases of Pants.

❌ 2.22.x

I was unable to cherry-pick this PR to 2.22.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.22.x \
     && git checkout -b cherry-pick-21568-to-2.22.x FETCH_HEAD \
     && git cherry-pick e18484d94ae835ddd7407bfba67c04c472785fb4

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "21568" "2.22.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

❌ 2.23.x

I was unable to cherry-pick this PR to 2.23.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.23.x \
     && git checkout -b cherry-pick-21568-to-2.23.x FETCH_HEAD \
     && git cherry-pick e18484d94ae835ddd7407bfba67c04c472785fb4

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "21568" "2.23.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

❌ 2.24.x

I was unable to cherry-pick this PR to 2.24.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.24.x \
     && git checkout -b cherry-pick-21568-to-2.24.x FETCH_HEAD \
     && git cherry-pick e18484d94ae835ddd7407bfba67c04c472785fb4

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "21568" "2.24.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

---

When you're done manually cherry-picking, please remove the needs-cherrypick label on this PR.

Thanks again for your contributions!

🤖 Beep Boop here's my run link