Fix URL joining in OptimadeRester

[ricardonpa]

Summary

Major changes:

• fix 1:
  • Fix URL joining in OptimadeRester. The original os.path.join function is not the correct function to use for joining URLs. It's designed for joining file paths, and its behavior depends on the operating system. On Windows, it uses a backslash () as the separator, while on Unix-based systems, it uses a forward slash (/).
  • This is a small fix, so no tests were added to 'test_optimade.py'
  • This fix should resolve the issue reported at: https://matsci.org/t/could-not-retrieve-information-from-optimade/39469

Todos

None

Checklist

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

[janosh]

thanks a lot @rdamaral for submitting a fix!