Some tests weren't using python3.

test/broker/09-extended-auth-change-username.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Check whether an extended auth plugin can change the username of a client.
 

test/broker/09-extended-auth-multistep-reauth.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from mosq_test_helper import *
 

test/broker/09-extended-auth-multistep.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 from mosq_test_helper import *
 

test/broker/09-extended-auth-single.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Multi tests for extended auth with a single step.
 # * Error in plugin

test/broker/09-extended-auth-single2.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Multi tests for extended auth with a single step - multiple plugins at once.
 # * Error in plugin

test/broker/09-extended-auth-unsupported.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Test whether an unsupported extended auth is rejected.