Use "macos-13" runner label in "pytest" workflow

[khaeru]

@glatterf42 noticed today that CI jobs were failing:

• For ixmp, message-ix, and message-ix-models.
• For macOS only.

…and traced this to a change in the standard GitHub Actions runners.

• macos-latest now runs on the M1/amd64 architecture.
• The amd64 architecture runners encounter the same problem discussed at IXMP on Apple Silicon (ARM architecture) #473 and more recently again in Slack.
• We looked at:
  • https://github.com/actions/runner-images/blame/a76eae469e4c8c16b0f91c38e17f9e1ffb5c633d/README.md#L26-L30
  • 2024 GitHub Actions macOS runner image roadmap actions/runner-images#9255
  • https://github.blog/changelog/2024-01-30-github-actions-macos-14-sonoma-is-now-available/
  • https://docs.github.com/en/actions/using-github-hosted-runners/about-larger-runners/about-larger-runners
  • https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories
• We interpret that:
  • For macOS, the only available options are (a) macos-14=macos-latest, which is on amd64, or (b) macos-14-large=macos-latest-large, which is on x86_64 but (per the above) not free.
  • The most recent macOS image that is both free and on x86_64 is macos-13.

This PR switches to macos-13 for these workflows.
NB I also had to change the branch protection rule for main to replace macos-latest-py3.12 as a required check with macos-13-py3.12.

• Another possible mitigation, but more work, is to get the workaround discussed in IXMP on Apple Silicon (ARM architecture) #473 to function on the GHA amd64 runners.
• In the future, macos-12 and eventually macos-13 labels will be deprecated and removed by GitHub. The above issue gives 2024-Q3 to deprecate and 2024-Q4 to remove macos-12, and has macos-13 still in general availability ("GA") through 2024-Q4.
• We would need to either do the above or get the test suite running with ixmp4 for macOS, i.e. via Implement ixmp4 shim #516 and other work.

How to review

Note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A
• Update release notes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (707353e) to head (1945ad0).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #527   +/-   ##
=====================================
  Coverage   98.9%   98.9%           
=====================================
  Files         44      44           
  Lines       4804    4804           
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  Hits        4754    4754           
  Misses        50      50           

[glatterf42]

Looks good, tests seem to be passing again. Thanks :)

[khaeru]

I left a longer discussion above so we can just cite this issue in PRs for the other two repos. Could you please open those?

At some point, we should open a PR (and maybe re-open #473) that updates the install instructions document a minimal work-around, maybe in the “Troubleshooting” section, for users who encounter that issue. As you noted, there are likely to be more in the future. We could then link to that work-around from the other install docs on the stack.

[glatterf42]

The message_ix "Install from conda-forge" workflow succeeds with macos-latest, so maybe this offers hints at a solution, too.