Fix test requirements

[mwouts]

and in particular

• require pandoc==2.16.2 in tests
• require a Python kernel for some tests

Relates to #906

[codecov]

Codecov Report

Merging #919 (6fbbee9) into main (a1795e0) will decrease coverage by 0.02%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##             main     #919      +/-   ##
==========================================
- Coverage   99.02%   99.00%   -0.03%     
==========================================
  Files         116      116              
  Lines       10780    10907     +127     
==========================================
+ Hits        10675    10798     +123     
- Misses        105      109       +4     

Impacted Files	Coverage Δ
tests/utils.py	100.00% <ø> (ø)
jupytext/pandoc.py	95.08% <80.00%> (-1.53%)	⬇️
tests/test_black.py	100.00% <100.00%> (ø)
tests/test_cli.py	99.11% <100.00%> (-0.89%)	⬇️
tests/test_execute.py	100.00% <100.00%> (ø)
jupytext/jupytext.py	98.59% <0.00%> (+0.19%)	⬆️
jupytext/__main__.py	100.00% <0.00%> (+100.00%)	⬆️

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