BLD: pin mpl-sphinx-theme to support sha in footer

Author: tacaswell

requirements/doc/doc-requirements.txt

@@ -14,7 +14,7 @@ ipywidgets
 numpydoc>=1.0
 packaging>=20
 pydata-sphinx-theme>=0.12.0
-mpl-sphinx-theme
+mpl-sphinx-theme>=3.7.0
 pyyaml
 sphinxcontrib-svg2pdfconverter>=1.1.0
 sphinx-gallery>=0.10