Merge pull request matplotlib#28193 from meeseeksmachine/auto-backport-of-pr-28185-on-v3.9.x

Backport PR matplotlib#28185 on branch v3.9.x (DOC: Bump mpl-sphinx-theme to 3.9)

Author: QuLogic

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-copybutton