Tests should work in the absence of a kernel

[mwouts]

Relates to #906

[codecov]

Codecov Report

Merging #921 (08f6ec0) into main (0182238) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #921   +/-   ##
=======================================
  Coverage   98.98%   98.99%           
=======================================
  Files         117      117           
  Lines       10854    10858    +4     
=======================================
+ Hits        10744    10749    +5     
+ Misses        110      109    -1     

Impacted Files	Coverage Δ
jupytext/version.py	100.00% <100.00%> (ø)
tests/test_contentsmanager.py	100.00% <100.00%> (ø)
tests/test_execute.py	100.00% <100.00%> (ø)
tests/test_using_cli.py	100.00% <100.00%> (ø)
jupytext/kernels.py	96.42% <0.00%> (+3.57%)	⬆️

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