Fix failing extension check tests

[mwouts]

This is a rebased version of #1297

[github-actions]

Thank you for making this pull request.

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

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

HATCH_BUILD_HOOKS_ENABLE=true pip install git+https://github.com/mwouts/jupytext.git@pr_1297_rebased

(this requires nodejs, see more at Developing Jupytext)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.04%. Comparing base (b3c8c50) to head (b249fb1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1313      +/-   ##
==========================================
+ Coverage   97.02%   97.04%   +0.02%     
==========================================
  Files          29       29              
  Lines        4499     4499              
==========================================
+ Hits         4365     4366       +1     
+ Misses        134      133       -1     

Flag	Coverage Δ
external	75.07% <ø> (?)
functional	88.39% <ø> (?)
integration	77.28% <ø> (?)
unit	66.48% <ø> (?)

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