Log Message:
py-pallets-sphinx-themes: updated to 2.1.3

Version 2.1.3
- Allow Sphinx's parallel build feature. :issue:`88`

Version 2.1.2
- Use modern packaging metadata with ``pyproject.toml`` instead of ``setup.cfg``.
- Use ``flit_core`` instead of ``setuptools`` as build backend.
- Compatibility with changes in Sphinx 7.3. :pr:`100`