Pin Sphinx libraries

requirements.txt

@@ -20,3 +20,10 @@ sphinxext-opengraph==0.5.1
 
 # Full-page search UI for RTD: https://readthedocs-sphinx-search.readthedocs.io
 readthedocs-sphinx-search==0.1.0
+
+# These get pulled in by Sphinx, we need to pin these as higher versions require Sphinx 5.0+.
+sphinxcontrib-applehelp<=1.0.4
+sphinxcontrib-htmlhelp<=2.0.1
+sphinxcontrib-qthelp<=1.0.3
+sphinxcontrib-serializinghtml<=1.1.5
+sphinxcontrib-devhelp<=1.0.2