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 #473 and more recently again in Slack.
• We looked at:
  • https://github.com/actions/runner-images/blame/a76eae469e4c8c16b0f91c38e17f9e1ffb5c633d/README.md#L26-L30
  • 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 #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 #516 and other work.

How to review

Note that the CI checks all pass.

PR checklist

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

[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.