upstream CI: Enable nightly tests using ansible-core 2.12.

This patch modifies the Python version used to be the latest available,
and add stages to execute the tests using ansible-core 2.12. As we
use Ubuntu 20.04, Python version 3.8 is avaiable.

Previously, ansible-core 2.12 was not available as it cannot be
installed with Python 3.6, which was the version used.

tests/azure/templates/playbook_tests.yml

@@ -14,7 +14,7 @@ parameters:
     default: ""
   - name: python_version
     type: string
-    default: 3.6
+    default: 3.x
   - name: build_number
     type: string
 

tests/azure/templates/pytest_tests.yml

@@ -9,7 +9,7 @@ parameters:
     default: ""
   - name: python_version
     type: string
-    default: 3.6
+    default: 3.x
 
 jobs:
 - job: Test_PyTests