feat: drop support of python 3.7

[frostming]

Signed-off-by: Frost Ming [email protected]

[codecov]

Codecov Report

Attention: Patch coverage is 41.66667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 96.21%. Comparing base (0963310) to head (5731dbc).
Report is 3 commits behind head on master.

Files	Patch %	Lines
marko/ast_renderer.py	0.00%	0 Missing and 3 partials ⚠️
marko/helpers.py	0.00%	0 Missing and 2 partials ⚠️
marko/source.py	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
- Coverage   96.22%   96.21%   -0.02%     
==========================================
  Files          21       21              
  Lines        2042     2035       -7     
  Branches      340      347       +7     
==========================================
- Hits         1965     1958       -7     
+ Misses         44       37       -7     
- Partials       33       40       +7     

Flag	Coverage Δ
unittests	96.21% <41.66%> (-0.02%)	⬇️

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