fix ReadTheDocs build: upgrade Python 3.9 → 3.11

sphinx-autodocgen requires Sphinx ≥8.0 which needs Python ≥3.10,
causing the RTD build to fail on Python 3.9. Also bump OS to
ubuntu-22.04.

Author: katosh

readthedocs.yaml

@@ -1,9 +1,9 @@
 version: 2
 
 build:
-  os: ubuntu-20.04
+  os: ubuntu-22.04
   tools:
-    python: "3.9"
+    python: "3.11"
 
 sphinx:
    configuration: docs/source/conf.py