MAINT: Docutils 0.18 deprecation

[larsoner]

Avoids a PendingDeprecationWarning:

Traceback (most recent call last):
  File "/home/larsoner/python/sphinx/sphinx/builders/html/__init__.py", line 1072, in handle_page
    output = self.templates.render(templatename, ctx)
...
  File "/home/larsoner/python/pydata-sphinx-theme/src/pydata_sphinx_theme/__init__.py", line 172, in generate_nav_html
    toc_sphinx = index_toctree(app, pagename, startdepth, **kwargs)
  File "/home/larsoner/python/pydata-sphinx-theme/src/pydata_sphinx_theme/__init__.py", line 428, in index_toctree
  File "/home/larsoner/python/pydata-sphinx-theme/src/pydata_sphinx_theme/__init__.py", line 385, in _get_local_toctree_for
    meth = 'findall' if hasattr(doctree, 'findall') else 'traverse'
  File "/home/larsoner/.local/lib/python3.10/site-packages/docutils/nodes.py", line 240, in traverse
    warnings.warn('nodes.Node.traverse() is obsoleted by Node.findall().',
PendingDeprecationWarning: nodes.Node.traverse() is obsoleted by Node.findall().