doc: fix .readthedocs.yaml build.tools.python

Hopefully fix the obscure error:

Problem in your project's configuration. Invalid "build.tools.python":
expected one of (2.7, 3.6, 3.7, 3.8, 3.9, 3.10, 3.11, pypy3.7, pypy3.8,
pypy3.9, miniconda3-4.7, mambaforge-4.10, 3), got 3.11

Author: jnikula

.readthedocs.yaml

@@ -4,7 +4,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: 3.11
+    python: "3.11"
   apt_packages:
     - python3-clang