Improve MD super block search robustness

[Schamper]

Fixes #42.

Also adds two annotation future imports I forgot to save with the Ruff change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.98%. Comparing base (b4afce2) to head (d56fe33).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   76.86%   76.98%   +0.12%     
==========================================
  Files          29       29              
  Lines        2377     2381       +4     
==========================================
+ Hits         1827     1833       +6     
+ Misses        550      548       -2     

Flag	Coverage Δ
unittests	76.98% <100.00%> (+0.12%)	⬆️

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