Adjust CI for Numpy 2.0, Graphviz

[khaeru]

This PR updates the "pytest" CI workflow to address two causes of failures:

• With NumPy 2.0.0 we run into Current version does not work with numpy 2.0 unionai-oss/pandera#1685, which is a dependency via message_ix.report → pyam-iamc → ixmp4. We apply the mitigation discussed at Adjust for NumPy 2.0.0 khaeru/genno#140.
• Brew attempts and fails to install python on macos-13 runners ts-graphviz/setup-graphviz#630. This mirrors changes made in Use pint != 0.24.0 in "pytest" CI workflow ixmp#535.

Both of these are temporary work-arounds, are marked accordingly, and should be removed when the respective upstream packages are fixed.

How to review

Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅ CI changes only.
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.6%. Comparing base (1125579) to head (c9df55e).
Report is 16 commits behind head on main.

Additional details and impacted files

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##            main    #854   +/-   ##
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  Coverage   95.6%   95.6%           
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  Files         46      46           
  Lines       4333    4333           
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  Hits        4143    4143           
  Misses       190     190