Compute the script extension for each file

[mwouts]

Fix #428

[codecov]

Codecov Report

Merging #430 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #430      +/-   ##
==========================================
+ Coverage   98.94%   98.94%   +<.01%     
==========================================
  Files          75       75              
  Lines        7645     7646       +1     
==========================================
+ Hits         7564     7565       +1     
  Misses         81       81

Impacted Files	Coverage Δ
jupytext/cli.py	95.62% <100%> (ø)	⬆️
tests/test_mirror.py	100% <100%> (ø)	⬆️
jupytext/version.py	100% <100%> (ø)	⬆️
jupytext/formats.py	98.67% <100%> (ø)	⬆️

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