Fix csv downloader for optuna v3.1.0

[keisuke-umezawa]

Contributor License Agreement

This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
Make sure that you understand the consequences concerning licenses and check the box below if you accept the term before creating this pull request.

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Reference Issues/PRs

#771 (comment)

What does this implement/fix? Explain your changes.

• Fix csv downloader for optuna v3.1.0
• Run unit tests of csv downloader with optuna v3.1.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (763e982) 69.38% compared to head (4550b8e) 69.39%.
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #781      +/-   ##
==========================================
+ Coverage   69.38%   69.39%   +0.01%     
==========================================
  Files          35       35              
  Lines        2368     2369       +1     
==========================================
+ Hits         1643     1644       +1     
  Misses        725      725              

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[c-bata]

LGTM!