Remove markdown-it-py pins

[philippjfr]

mdit-py-plugins compatible with markdown-it-py==3 was released.

[codecov]

Codecov Report

Merging #5159 (1fcf191) into main (0fc20f3) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5159      +/-   ##
==========================================
+ Coverage   83.59%   83.72%   +0.12%     
==========================================
  Files         274      274              
  Lines       39498    39498              
==========================================
+ Hits        33020    33070      +50     
+ Misses       6478     6428      -50     

Flag	Coverage Δ
ui-tests	40.52% <100.00%> (+0.99%)	⬆️
unitexamples-tests	73.69% <0.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
panel/io/convert.py	72.18% <100.00%> (ø)

... and 12 files with indirect coverage changes

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