Merge ~cjwatson/launchpad-buildd:readthedocs-config into launchpad-buildd:master

[Colin Watson]

Refine readthedocs configuration

This applies some post-merge review suggestions from https://code.launchpad.net/~cjwatson/launchpad-buildd/+git/launchpad-buildd/+merge/444726.

[Jürgen Gmach (jugmac00)]

review: Approve

diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index faed046..a95166c 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -3,7 +3,8 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.11"
+    python: "3"
 
 sphinx:
   configuration: docs/conf.py
+  fail_on_warning: true