Pin python version in getting started tutorial

tutorials/katacoda/getting-started/src/tekton-katacoda/tasks/build.yaml

@@ -26,7 +26,7 @@ spec:
     default: app
   steps:
   - name: pytest
-    image: python
+    image: python:3.9
     command:
     - /bin/bash
     - -c

tutorials/katacoda/getting-started/tasks.md

@@ -63,7 +63,7 @@ step to `spec.steps`:
 steps:
 # The name of the step
 - name: pytest
-  image: python
+  image: python:3.9
   command:
   - /bin/bash
   - -c