WIP Cell ids for text notebooks are deterministic

[mwouts]

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This PR was initially motivated by #941, but in the end it has been superseded by #945 which is a way more direct solution. Nevertheless the need for non-random cell ids on text documents may still be there, see e.g. #922

[codecov[bot]]

Codecov Report

Merging #944 (d6b4ec1) into main (8d48e18) will decrease coverage by 0.00%. The diff coverage is 96.87%.

@@            Coverage Diff             @@
##             main     #944      +/-   ##
==========================================
- Coverage   99.07%   99.06%   -0.01%     
==========================================
  Files         117      118       +1     
  Lines       10867    10884      +17     
==========================================
+ Hits        10766    10782      +16     
- Misses        101      102       +1     

Impacted Files	Coverage Δ
tests/conftest.py	100.00% <ø> (ø)
jupytext/jupytext.py	98.04% <83.33%> (-0.36%)	:arrow_down:
jupytext/cell_ids.py	100.00% <100.00%> (ø)
tests/test_cell_id.py	100.00% <100.00%> (ø)
tests/test_cli.py	100.00% <100.00%> (ø)

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[mwouts]

I am wondering whether the cell id generator should be independent of the YAML header, or even if it should not depend at all on the notebook (but then, the sequence of cell ids will be the same for all text notebooks)...