Update .readthedocs.yaml Python version

As per readthedocs/readthedocs.org#9081 - while the template suggests Python 3.11, this doesn't actually work with the version of mkdocs installed by default. 

Reverting to Python 3.9 apparently fixes it.

Author: ProfYaffle

.readthedocs.yaml

@@ -11,7 +11,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.11"
+    python: "3.9"
 
 mkdocs:
   configuration: mkdocs.yml