Add python version to Azure PR pipeline

Author: stevenmccullaghmadetech

azure/azure-pr-pipeline.yml

@@ -29,6 +29,7 @@ variables:
 extends:
   template: azure/common/apigee-deployment.yml@common
   parameters:
+    python_version: 3.9
     service_name: ${{ variables.service_name }}
     service_base_path: ${{ variables.service_base_path }}
     short_service_name: ${{ variables.short_service_name }}