Use Python 3.12 for readthedocs

I suspect there's an issue affecting docutils or a related tool on Python < 3.12. My guess is that whatever causes flake8 to crash is also stopping sphinx from building. Sphinx builds locally on 3.12, so I'm committing this to try it on readthedocs.

Author: rwb27

.readthedocs.yaml

@@ -8,7 +8,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.11"
+    python: "3.12"
 
 # Build documentation in the "docs/" directory with Sphinx
 sphinx: