Skip the tests that execute notebooks on Windows

[mwouts]

• Skip the tests that execute a notebook on Windows to avoid timeout issues (close Intermittent error when testing "jupytext --execute" on Windows #489)
• Target 100% coverage in tests

[codecov]

Codecov Report

Merging #579 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #579      +/-   ##
==========================================
+ Coverage   98.90%   98.99%   +0.08%     
==========================================
  Files          89       89              
  Lines        8802     8794       -8     
==========================================
  Hits         8706     8706              
+ Misses         96       88       -8     

Impacted Files	Coverage Δ
tests/test_pep8.py	100.00% <ø> (+1.61%)	⬆️
tests/test_cli.py	100.00% <100.00%> (+0.32%)	⬆️
tests/test_combine.py	100.00% <100.00%> (+2.70%)	⬆️
tests/test_execute.py	100.00% <100.00%> (+2.40%)	⬆️
tests/utils.py	100.00% <100.00%> (ø)
jupytext/formats.py	97.20% <0.00%> (+0.31%)	⬆️

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