Set build.os on readthedocs

Readthedocs now requires an explicit image [1] to build.

> We are announcing the deprecation of build.image config key in favor
> of build.os. Read the Docs will start requiring a build.os config key
> for all projects in order to build documentation successfully. We will
> start failing builds for projects not using “build.os” in their config
> file on October 16, 2023.

[1] https://blog.readthedocs.com/use-build-os-config/

.readthedocs.yaml

@@ -5,6 +5,11 @@
 # Required
 version: 2
 
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.11"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
    configuration: docs/source/conf.py