Remove: 移除 Python 3.8 支持

[yanyongyu]

No description provided.

[github-actions]

🚀 Deployed on https://deploy-preview-2641--nonebot2.netlify.app

[codecov]

Codecov Report

Attention: Patch coverage is 99.28826% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 94.22%. Comparing base (e93ee1f) to head (4554f83).

Files	Patch %	Lines
nonebot/drivers/aiohttp.py	75.00%	1 Missing ⚠️
nonebot/utils.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2641      +/-   ##
==========================================
+ Coverage   94.14%   94.22%   +0.08%     
==========================================
  Files          48       48              
  Lines        3997     4019      +22     
==========================================
+ Hits         3763     3787      +24     
+ Misses        234      232       -2     

Flag	Coverage Δ
unittests	94.22% <99.28%> (+0.08%)	⬆️

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