Jupytext refuses to open a non-unicode file (api, cli & jupyter)

[mwouts]

Fixes #896

[codecov]

Codecov Report

Merging #897 (dc468aa) into main (f843c54) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #897   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files         114      115    +1     
  Lines       10699    10721   +22     
=======================================
+ Hits        10597    10619   +22     
  Misses        102      102           

Impacted Files	Coverage Δ
jupytext/contentsmanager.py	97.61% <100.00%> (ø)
jupytext/version.py	100.00% <100.00%> (ø)
tests/test_invalid_file.py	100.00% <100.00%> (ø)

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[drdarshan]

Thank you for the quick fix @mwouts!