Reset label for sites changed by Structure.replace_species()

[stefsmeets]

Summary

This PR resets the label on any site changed by Structure.replace_species(). Site.label will fall back to use the default label instead (the species string).

Closes #3658

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.

Tip: Install pre-commit hooks to auto-check types and linting before every commit:

pip install -U pre-commit
pre-commit install

[janosh]

thanks a lot @stefsmeets. you have a knack for fixing stuff with minimal diff. 👍 😄