Work around JPype1 1.4.1 issues in "pytest" CI workflow

[khaeru]

This is a mitigation (not a fix) for #463.

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

Merging #464 (2d01b85) into main (0c2d7de) will decrease coverage by 0.0%. The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #464     +/-   ##
=======================================
- Coverage   98.6%   98.5%   -0.1%     
=======================================
  Files         42      42             
  Lines       4468    4468             
=======================================
- Hits        4406    4404      -2     
- Misses        62      64      +2     

Impacted Files	Coverage Δ
ixmp/utils/__init__.py	98.6% <0.0%> (-1.0%)	:arrow_down: