[Bug] FutureWarning: Series.__getitem__ treating keys as positions is deprecated

[Hirokazu Takaya]

Checkbox

• I agree to follow code of conduct and to engage in discussion actively.
• I agree to create a new issue with the other templates if this is not a bug.
• I used the latest version of covsirphy.

Summary

Test workflow raised the following warning.
https://github.com/lisphilar/covid19-sir/actions/runs/6064514921/job/16453042386?pr=1512

FutureWarning: Series.__getitem__ treating keys as positions is deprecated. In a future version, integer keys will always be treated as labels (consistent with DataFrame behavior). To access a value by position, use `ser.iloc[pos]`

Reproducible example script

NA

The current outputs

NA

Expected outputs

No errors.

Environment

- CovsirPhy version: 3.0.0
- Python version: 3.11.0

Package manager (required if installation issue)

No response

Platform (required if installation issue)

No response

Additional Context

No response