Explicitly use python3 interpreter

controller/python/check-firmware.py

+#! /usr/bin/env python3
+
 # Check that the microcontroller behaves as expected by exchanging
 # information with the microcontroller.
 

controller/python/check-wiring.py

-#! /usr/bin/env python
+#! /usr/bin/env python3
 
 import controller
 import time

controller/python/localpid.py

-#! /usr/bin/env python
+#! /usr/bin/env python3
 #
 # Basic PID test
 

controller/python/showencoders.py

-#! /usr/bin/env python
+#! /usr/bin/env python3
 #
 # Show encoders value as fast as possible