Set build.os in .readthedocs.yml

This is now required, apparently.

See:
- readthedocs/readthedocs.org#8861 (comment)
- readthedocs/readthedocs.org#8912

(cherry picked from commit JonathonReinhart/scuba@4b89e6e178)

Author: JonathonReinhart

.readthedocs.yaml

@@ -1,6 +1,11 @@
 # https://docs.readthedocs.io/en/stable/config-file/v2.html
 version: 2
 
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.10"
+
 sphinx:
   configuration: docs/conf.py