Adapt .readthedocs.yml to use build.os instead of build.image

build.image deprecated (https://blog.readthedocs.com/use-build-os-config/). adapted some other lines according to documentation (https://docs.readthedocs.io/en/stable/config-file/v2.html#build)

.readthedocs.yml

@@ -1,13 +1,18 @@
 # Required
 version: 2
 
-# Image to use
+# Set OS and Python version
 build:
-  image: testing
+  os: ubuntu-22.04
+  tools:
+    python: "3.9"
+
+# Build documentation in the "docs/" directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
 
 # Configuration
 python:
-  version: 3.9
   install:
     - requirements: docs/requirements.txt
     - method: pip