[Bug] FutureWarning: Setting an item of incompatible dtype 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/6065286431/job/16454781182?pr=1512

FutureWarning: Setting an item of incompatible dtype is deprecated and will raise in a future error of pandas. Value 'Grand Princess' has dtype incompatible with float64, please explicitly cast to a compatible dtype first.

Reproducible example script

NA

The current outputs

No response

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