Use python3 as the canonical command name course-wide (#104) (#105)

modules/18-continuous-delivery-and-deployment/README.md

@@ -250,7 +250,7 @@ A CLI that exits immediately is awkward to "deploy." Give the app a long-running
 2. Run the service locally first, no container, to see it work:
 
    ```bash
-   python serve.py        # serves on http://localhost:8000
+   python3 serve.py        # serves on http://localhost:8000
    ```
 
    In another terminal:

modules/18-continuous-delivery-and-deployment/lab/serve.py

@@ -4,7 +4,7 @@ Standard library only, no pip install, so the container image stays tiny and the
 dependencies to drift. It reuses the TaskList from tasks.py (Modules 1-2) unchanged.
 
 Run it:
-    python serve.py            # serves on http://localhost:8000
+    python3 serve.py            # serves on http://localhost:8000
 
 Endpoints:
     GET /health   -> {"status": "ok", "version": <APP_VERSION>}   (200)