Upgrade Python version from 3.8 to 3.9

Author: Mariosmsk

.readthedocs.yaml

@@ -9,7 +9,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.8"
+    python: "3.9"
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
@@ -19,4 +19,4 @@ sphinx:
 # https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 python:
   install:
-  - requirements: docs/requirements.txt
+  - requirements: docs/requirements.txt