fix: update Python version from 3.8 to 3.9 in Azure pipeline configuration

Author: eleanorjboyd

build/azure-pipeline.stable.yml

@@ -61,7 +61,7 @@ extends:
 
       - task: UsePythonVersion@0
         inputs:
-          versionSpec: '3.8'
+          versionSpec: '3.9'
           addToPath: true
           architecture: 'x64'
         displayName: Select Python version