Ignore changes in the YAML header in jupytext --test

[mwouts]

Closes #414

[codecov]

Codecov Report

Merging #420 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #420      +/-   ##
==========================================
+ Coverage   98.93%   98.93%   +<.01%     
==========================================
  Files          75       75              
  Lines        7596     7608      +12     
==========================================
+ Hits         7515     7527      +12     
  Misses         81       81

Impacted Files	Coverage Δ
jupytext/cli.py	95.62% <100%> (+0.08%)	⬆️
tests/test_cli.py	100% <100%> (ø)	⬆️
tests/test_read_simple_pandoc.py	100% <0%> (ø)	⬆️
jupytext/pandoc.py	95.91% <0%> (ø)	⬆️
jupytext/jupytext.py	98.59% <0%> (ø)	⬆️
jupytext/kernels.py	90% <0%> (ø)	⬆️
tests/test_contentsmanager.py	100% <0%> (ø)	⬆️
tests/test_mirror.py	100% <0%> (ø)	⬆️
jupytext/formats.py	98.66% <0%> (ø)	⬆️

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