Docs: update readthedocs.yml config

Move deprecated python.version to build.tools.python,
use latest stable Python 3 (rather than pinning 3.7).

.readthedocs.yml

@@ -3,6 +3,12 @@
 
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    # "last stable CPython version":
+    python: "3"
+
 sphinx:
   configuration: docs/conf.py
   builder: dirhtml
@@ -11,6 +17,5 @@ sphinx:
 formats: all
 
 python:
-  version: 3.7
   install:
     - requirements: docs/requirements.txt