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

blog/11-reviewing-code-you-didnt-write.md

@@ -55,7 +55,7 @@ And here's the part people resist: this holds **even when you're the only human
 Talk is cheap, so here's the lab the course runs, compressed. You've got a tiny `tasks-app`, a command-line to-do list. In the base version, `complete()` validates the index, so `done 99` on a list with three tasks gives you a clean, loud error and a non-zero exit code:
 
 ```bash
-python cli.py done 99        # prints "error: no task at index 99", exits non-zero
+python3 cli.py done 99        # prints "error: no task at index 99", exits non-zero
 echo "exit code: $?"
 ```
 
@@ -74,7 +74,7 @@ The diff adds a `delete` command. It works: try `delete 0`, the task goes away,
 But run the *failure* path, not the happy one:
 
 ```bash
-python cli.py done 99         # the trap
+python3 cli.py done 99         # the trap
 echo "exit code: $?"
 ```