Add workflow to upload package to TestPyPI

[tky823]

Summary

This PR adds a GHA to upload the package to TestPyPI.

ref #250

Note

This workflow is a dummy to confirm its usage.

[codecov-commenter]

Codecov Report

Patch coverage: 55.55% and project coverage change: -0.07% :warning:

Comparison is base (2ca9933) 94.02% compared to head (4646e3a) 93.95%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #252 +/- ## ========================================== - Coverage 94.02% 93.95% -0.07% ========================================== Files 44 44 Lines 5403 5409 +6 ========================================== + Hits 5080 5082 +2 - Misses 323 327 +4 ``` | [Files Changed](https://app.codecov.io/gh/tky823/ssspy/pull/252?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [ssspy/\_\_init\_\_.py](https://app.codecov.io/gh/tky823/ssspy/pull/252?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3NzcHkvX19pbml0X18ucHk=) | `55.55% <55.55%> (-44.45%)` | :arrow_down: |

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[tky823]

Since workflow_run only supports a workflow run on the default branch, the uploading workflow was not triggered. https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_run

[tky823]

I checked the behavior of the automatic package upload, so I'll close this PR.