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 unionai-oss/pandera#1685, which is a dependency via message_ix.report → pyam-iamc → ixmp4. We apply the mitigation discussed at khaeru/genno#140.
• ts-graphviz/setup-graphviz#630. This mirrors changes made in iiasa/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

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

[codecov[bot]]

Codecov Report

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

Project coverage is 95.6%. Comparing base (1125579) to head (c9df55e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #854 +/- ## ===================================== Coverage 95.6% 95.6% ===================================== Files 46 46 Lines 4333 4333 ===================================== Hits 4143 4143 Misses 190 190 ```