[MAINT] update RTD config

.readthedocs.yml

@@ -1,10 +1,14 @@
----
 # Read the Docs configuration file
 # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
 
 # Required
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: '3.12'
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/source/conf.py
@@ -13,10 +17,9 @@ sphinx:
 
 # Optionally build your docs in additional formats such as PDF
 formats:
-- pdf
+  - pdf
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.8
   install:
-  - requirements: requirements.txt
+    - requirements: requirements.txt