Normalize the sample notebooks

[mwouts]

Fixes #1002

[codecov]

Codecov Report

Merging #1003 (a9a5f7d) into main (12964ef) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1003   +/-   ##
=======================================
  Coverage   99.06%   99.06%           
=======================================
  Files         118      119    +1     
  Lines       11021    11029    +8     
=======================================
+ Hits        10918    10926    +8     
  Misses        103      103           

Impacted Files	Coverage Δ
jupytext/version.py	100.00% <100.00%> (ø)
tests/test_jupytext_nbconvert_round_trip.py	100.00% <100.00%> (ø)
tests/test_sample_notebooks_are_normalized.py	100.00% <100.00%> (ø)
tests/utils.py	100.00% <100.00%> (ø)

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