Build docs with 3.11 on ReadTheDocs

Inspired by https://github.com/simonw/sqlite-utils/issues/540

.readthedocs.yaml

@@ -3,7 +3,7 @@ version: 2
 build:
   os: ubuntu-20.04
   tools:
-    python: "3.9"
+    python: "3.11"
 
 sphinx:
    configuration: docs/conf.py