Ignore showIndex when df is a Polars DataFrame

[mwouts]

Closes #422

[github-actions]

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

pip install git+https://github.com/mwouts/itables.git@version_2.4.5

(this requires nodejs, see more at Developing ITables)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.11%. Comparing base (8870083) to head (2ad4f0c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #423      +/-   ##
==========================================
+ Coverage   88.08%   88.11%   +0.03%     
==========================================
  Files          50       50              
  Lines        2014     2020       +6     
==========================================
+ Hits         1774     1780       +6     
  Misses        240      240              

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