Fix build failure with newer sphinx dependencies:

[LocutusOfBorg]

=================================== FAILURES =================================== __________________________ test_sd_custom_directives ___________________________

sphinx_builder = <function sphinx_builder.._create_project at 0x73427f5c0900> file_regression = <pytest_regressions.file_regression.FileRegressionFixture object at 0x73427f8ba2c0>

@pytest.mark.skipif(not MYST_INSTALLED, reason="myst-parser not installed")
def test_sd_custom_directives(
    sphinx_builder: Callable[..., SphinxBuilder], file_regression
):
    """Test that the defaults are used."""
    builder = sphinx_builder(
        conf_kwargs={
            "extensions": ["myst_parser", "sphinx_design"],
            "sd_custom_directives": {
                "dropdown-syntax": {
                    "inherit": "dropdown",
                    "argument": "Syntax",
                    "options": {
                        "color": "primary",
                        "icon": "code",
                    },
                }
            },
        }
    )
    content = "# Heading\n\n```{dropdown-syntax}\ncontent\n```"
    builder.src_path.joinpath("index.md").write_text(content, encoding="utf8")
    builder.build()
    doctree = builder.get_doctree("index", post_transforms=False)
    doctree.attributes.pop("translation_progress", None)  # added in sphinx 7.1

  file_regression.check(

        doctree.pformat(),
        basename="sd_custom_directives",
        extension=".xml",
        encoding="utf8",
    )

E AssertionError: FILES DIFFER:
E /tmp/pytest-of-pbuilder/pytest-0/test_sd_custom_directives0/test_snippets/sd_custom_directives.xml
E /tmp/pytest-of-pbuilder/pytest-0/test_sd_custom_directives0/test_snippets/sd_custom_directives.obtained.xml
E HTML DIFF: /tmp/pytest-of-pbuilder/pytest-0/test_sd_custom_directives0/test_snippets/sd_custom_directives.obtained.diff.html
E ---
E +++
E @@ -2,7 +2,7 @@
E

E <title>
E Heading
E -
E +
E
E Syntax
E

tests/test_snippets.py:190: AssertionError
=============================== warnings summary =============================== tests/test_snippets.py::test_sd_hide_title_rst
tests/test_snippets.py::test_sd_hide_title_myst
tests/test_snippets.py::test_sd_custom_directives