Support markdown-it-py 4

[gaborbernat]

Resolves #533

Ran the CI here https://github.com/gaborbernat/mdformat/actions/runs/17328552712?pr=1 and seems to be working fine.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (499b47a) to head (b33c126).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #534   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          970       970           
  Branches       171       171           
=========================================
  Hits           970       970           

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[hukkin]

Thanks!

[gaborbernat]

@hukkin can we release this? Makes using mdformat impossible with rich as that one requires >3. So have to use older version that's causing issues.

[hukkin]

See #545 (comment)

I want to do this ASAP, but a little more than a push of a button is required this time so I haven't found the time yet.