Fix the generation of the "multiindex" polars df

[mwouts]

Cf. pola-rs/polars#18130

[github-actions]

Thank you for making this pull request.

Did you know? You can try it on Binder: .

Also, the version of ITables developed in this PR can be installed with pip:

pip install git+https://github.com/mwouts/itables.git@fix_ci

(this requires nodejs, see more at Developing ITables)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.71%. Comparing base (dfe261c) to head (4c7548e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #308      +/-   ##
==========================================
- Coverage   96.19%   95.71%   -0.49%     
==========================================
  Files          27       27              
  Lines        1183     1189       +6     
==========================================
  Hits         1138     1138              
- Misses         45       51       +6     

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