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

[lisphilar]

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

Tests failed with the following error.

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

No response

Expected outputs

No response

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