Update .readthedocs.yaml

Looks like the file is too old and requires modernization in order to
work with current version of readthedocs.

.readthedocs.yml

@@ -1,9 +1,16 @@
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.12"
+
 sphinx:
   configuration: docs/conf.py
 
 python:
-  version: 3.6
   install:
     - requirements: docs/requirements.txt