Update callouts.py for python3

Specify that python3 should be used in the script.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I826f31b376dae484c52a244c9fb7dc9fb8f917c1
diff --git a/callouts/callouts.py b/callouts/callouts.py
index c9a5024..3b59587 100755
--- a/callouts/callouts.py
+++ b/callouts/callouts.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import yaml