Make strikethrough precedence the default; add strikethrough tests

[KevinMGranger]

I noticed that strikethroughs were parsing incorrectly. If they have lower precedence than a link, then they'll break both themselves and the link!

I added a test for that case, as well as for examples from the GFM spec. I also added python 3.11 to the list of nox pythons.

The example for triple-strikes not changing into <del> still doesn't pass, but that seems more involved to fix, and should be a separate issue / PR.

[codecov]

Codecov Report

Base: 95.31% // Head: 95.47% // Increases project coverage by +0.15% 🎉

Coverage data is based on head (6161223) compared to base (bafe94d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
+ Coverage   95.31%   95.47%   +0.15%     
==========================================
  Files          19       19              
  Lines        1877     1877              
  Branches      320      320              
==========================================
+ Hits         1789     1792       +3     
+ Misses         46       44       -2     
+ Partials       42       41       -1     

Flag	Coverage Δ
unittests	95.47% <100.00%> (+0.15%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
marko/ext/gfm/elements.py	81.87% <100.00%> (+1.25%)	⬆️
marko/ext/gfm/__init__.py	88.63% <0.00%> (+2.27%)	⬆️

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