Bump Python version for readthedocs build

* Need to bump it because pillow~=12.0.0 does not support lower Python versions

Author: pushfoo

.readthedocs.yaml

@@ -10,7 +10,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.9"
+    python: "3.12"
 
 # Build documentation in the "docs/" directory with Sphinx
 sphinx:
@@ -32,4 +32,4 @@ sphinx:
 
 python:
     install:
-    - requirements: docs/requirements.txt
+    - requirements: docs/requirements.txt