Adjust CI for numpy 2.0, graphviz, pint

[khaeru]

This PR updates the "pytest" CI workflow to address three 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 various reporting code → pyam-iamc → ixmp4. We apply the mitigation discussed at Adjust for NumPy 2.0.0 khaeru/genno#140. This is parallel to Adjust CI for Numpy 2.0, Graphviz message_ix#854.
• Default format of dimensionless Unit: undocumented change in 0.24 hgrecco/pint#2007; see Work around bug in pint 0.24.0 ixmp#534.
• 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 and Adjust CI for Numpy 2.0, Graphviz message_ix#854.

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 52.1%. Comparing base (c6285cf) to head (6733914).
Report is 1810 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #200   +/-   ##
=====================================
  Coverage   52.1%   52.1%           
=====================================
  Files        141     141           
  Lines      11335   11338    +3     
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+ Hits        5914    5917    +3     
  Misses      5421    5421           

Files with missing lines	Coverage Δ
message_ix_models/tests/test_workflow.py	100.0% <100.0%> (ø)