feat: drop python 3.7 support, upgrade the syntax

[frostming]

No description provided.

[codecov]

Codecov Report

Base: 95.47% // Head: 95.48% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (8420af9) compared to base (3793819).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   95.47%   95.48%   +0.01%     
==========================================
  Files          18       18              
  Lines        1811     1816       +5     
  Branches      311      311              
==========================================
+ Hits         1729     1734       +5     
  Misses         42       42              
  Partials       40       40              

Flag	Coverage Δ
unittests	95.48% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
marko/ext/gfm/__init__.py	86.36% <ø> (ø)
marko/html_renderer.py	100.00% <ø> (ø)
marko/ast_renderer.py	91.22% <100.00%> (+0.15%)	⬆️
marko/block.py	96.54% <100.00%> (-0.01%)	⬇️
marko/ext/codehilite.py	100.00% <100.00%> (ø)
marko/ext/footnote.py	95.45% <100.00%> (ø)
marko/ext/gfm/elements.py	80.62% <100.00%> (ø)
marko/ext/latex_renderer.py	100.00% <100.00%> (ø)
marko/helpers.py	98.70% <100.00%> (ø)
marko/inline.py	100.00% <100.00%> (ø)
... and 3 more

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