Create prefix dir before moving paired notebooks

[mwouts]

Closes #1059

[codecov]

Codecov Report

Merging #1082 (d236736) into main (64b84b7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1082   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files         120      120           
  Lines       11120    11152   +32     
=======================================
+ Hits        11014    11046   +32     
  Misses        106      106           

Impacted Files	Coverage Δ
jupytext/cell_metadata.py	98.99% <100.00%> (ø)
jupytext/contentsmanager.py	95.81% <100.00%> (+0.01%)	⬆️
jupytext/formats.py	98.46% <100.00%> (+<0.01%)	⬆️
jupytext/version.py	100.00% <100.00%> (ø)
tests/test_contentsmanager.py	100.00% <100.00%> (ø)
tests/test_read_simple_markdown.py	100.00% <100.00%> (ø)
tests/test_read_simple_rmd.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more