The model timestamp of a paired notebook is the timestamp of the input file

[mwouts]

This was the case previously when content=False only. Now it will always be the case.
Closes #978 .

[codecov]

Codecov Report

Merging #983 (b2c481a) into main (1ca33a2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head b2c481a differs from pull request most recent head 869e90a. Consider uploading reports for the commit 869e90a to get more accurate results

@@           Coverage Diff           @@
##             main     #983   +/-   ##
=======================================
  Coverage   99.06%   99.06%           
=======================================
  Files         118      118           
  Lines       11004    11021   +17     
=======================================
+ Hits        10901    10918   +17     
  Misses        103      103           

Impacted Files	Coverage Δ
jupytext/contentsmanager.py	95.80% <100.00%> (-0.03%)	⬇️
jupytext/version.py	100.00% <100.00%> (ø)
tests/test_contentsmanager.py	100.00% <100.00%> (ø)

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