Fix xsign not found error

Install Python 3 and update to MicroBuild signing plugin v4 to fix
the errors:

Unable to load extension 'xsignextension: No module named 'azure.keyvault.key_vault_client''. Use --debug for more information.
'xsign' is misspelled or not recognized by the system.

azure-pipelines.yml

@@ -50,7 +50,12 @@ extends:
 
         - task: CodeQL3000Init@0
 
-        - task: MicroBuildSigningPlugin@3
+        # Python 3 is needed for the MicroBuild Signing Plugin
+        - task: UsePythonVersion@0
+          inputs:
+            versionSpec: '3.11'
+
+        - task: MicroBuildSigningPlugin@4
           displayName: 'Install Signing Plugin'
           inputs:
             signType: 'Real'