Remove 'unsafe' loader type from YAML in DftSet to avoid PendingDepre…

[cogsworth37]

…cationWarning

Summary

Major changes:

fixes materialsproject/pymatgen#4444

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

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

pip install -U pre-commit
pre-commit install