Use a fixed container image version for Python

The version of `pytest` specified in `dev_requirements.txt` is incompatible with Python 3.10. In order to make the Katacoda experience as reproducible as possible, it’s a good practice to use fixed versions for as many things as possible anyway.

tutorials/katacoda/tasks/src/tekton-katacoda/tasks/task.yaml

@@ -9,7 +9,7 @@ spec:
       type: git
   steps:
   - name: pytest
-    image: python
+    image: python:3.9
     command:
     - /bin/bash
     - -c