DOC: bump RTD python version

Currently running into this bug: readthedocs/sphinx_rtd_theme#1115

Hoping bumping the python version alone is sufficient, rather than
specifying build dep versions.

fix #273

.readthedocs.yaml

@@ -5,7 +5,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.10"
+    python: "3.11"
   jobs:
     post_install:
       - pip install ".[docs]"