docs: address RTD build deprecation

Switch to 'build.os' instead of 'build.image' to address the deprecation
of the 'build.image' config key in Read the Docs. This change is
necessary for successful documentation building.

.readthedocs.yml

@@ -3,11 +3,12 @@ version: 2
 
 # Image to use
 build:
-  image: testing
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.9"
 
 # Configuration
 python:
-  version: 3.9
   install:
     - requirements: docs/requirements.txt
     - method: pip