Fix: 消息段数据采用 str 存储

[yanyongyu]

related to #95

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 66.96%. Comparing base (8151707) to head (430aba0).

Files	Patch %	Lines
nonebot/adapters/onebot/v11/message.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
+ Coverage   66.92%   66.96%   +0.03%     
==========================================
  Files          24       24              
  Lines        2156     2155       -1     
==========================================
  Hits         1443     1443              
+ Misses        713      712       -1     

Flag	Coverage Δ
unittests	66.96% <50.00%> (+0.03%)	⬆️

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