use python 3.11 for readthedocs

3.9 fails to build environment there

Author: emanuel-schmid

.readthedocs.yml

@@ -11,7 +11,7 @@ build:
   # Append fixed Python version to requirements
   jobs:
     pre_create_environment:
-      - echo "  - python=3.9" >> requirements/env_climada.yml
+      - echo "  - python=3.11" >> requirements/env_climada.yml
 
 conda:
   environment: requirements/env_climada.yml