Fix .readthedocs.yml file

Readthedocs build automation has a new format. This patch fixes build
errors for https://test-definitions.readthedocs.io/

Signed-off-by: Milosz Wasilewski <milosz.wasilewski@foundries.io>

Author: mwasilew

.readthedocs.yml

@@ -1,8 +1,11 @@
 version: 2
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.7"
 mkdocs:
   configuration: mkdocs.yml
 formats: all
 python:
-  version: 3.7
   install:
     - requirements: mkdocs_plugin/requirements.txt