Explicit dependency on mdit-py-plugins

[mwouts]

As suggested by @kloczek in this comment

@kloczek, @chrisjsewell , does this PR sounds right for each of you? Thanks

[chrisjsewell]

Yep no problem 👍

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@70f0a96). Click here to learn what that means.
The diff coverage is 97.55%.

@@            Coverage Diff            @@
##             master     #822   +/-   ##
=========================================
  Coverage          ?   99.10%           
=========================================
  Files             ?      106           
  Lines             ?    10299           
  Branches          ?        0           
=========================================
  Hits              ?    10207           
  Misses            ?       92           
  Partials          ?        0           

Impacted Files	Coverage Δ
jupytext/reraise.py	25.00% <25.00%> (ø)
jupytext/kernels.py	92.85% <92.85%> (ø)
jupytext/pep8.py	95.58% <95.58%> (ø)
jupytext/cli.py	96.10% <96.04%> (ø)
jupytext/cell_reader.py	96.86% <96.49%> (ø)
jupytext/pandoc.py	96.61% <96.61%> (ø)
jupytext/cell_to_text.py	97.80% <97.16%> (ø)
jupytext/myst.py	97.19% <97.19%> (ø)
jupytext/config.py	97.28% <97.28%> (ø)
jupytext/contentsmanager.py	97.85% <97.73%> (ø)
... and 17 more

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[kloczek]

Yep .. thank you :)