Backport PR matplotlib#28185: DOC: Bump mpl-sphinx-theme to 3.9

Author: timhoffm

requirements/doc/doc-requirements.txt

@@ -15,7 +15,7 @@ ipykernel
 numpydoc>=1.0
 packaging>=20
 pydata-sphinx-theme~=0.15.0
-mpl-sphinx-theme~=3.8.0
+mpl-sphinx-theme~=3.9.0
 pyyaml
 sphinxcontrib-svg2pdfconverter>=1.1.0
 sphinx-gallery>=0.12.0