improve ux when trying to open sqlite without protocol

[turbotimon]

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This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
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• I agree this patch may be ported to Goptuna by other Goptuna contributors.

Reference Issues/PRs

Fixes/Improves UX for #799

What does this implement/fix? Explain your changes.

• Improves UX by guessing file using file header to open a sqlite db without protocol, e.g.:

$ optuna-dashboard example.db

• flake8 and mypy are happy with my changes
• i tried to add tests, but was not able because pytest python_test/ fails on main with import errors

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (5a4070c) 68.96% compared to head (d425b2f) 69.78%.
Report is 78 commits behind head on main.

Files	Patch %	Lines
optuna_dashboard/_app.py	78.57%	3 Missing ⚠️
optuna_dashboard/_storage_url.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #800      +/-   ##
==========================================
+ Coverage   68.96%   69.78%   +0.81%     
==========================================
  Files          35       35              
  Lines        2388     2373      -15     
==========================================
+ Hits         1647     1656       +9     
+ Misses        741      717      -24     

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Since the PR was updated.

[c-bata]

Let me assign @keisuke-umezawa as a reviewer.

@turbotimon Could you fix a flake8 error?

[turbotimon]

Let me assign @keisuke-umezawa as a reviewer.

@turbotimon Could you fix a flake8 error?

Done!

[keisuke-umezawa]

LGTM! It also works in my env.