test: Make exceptions compatible with Python 3

Use "except Type as var" syntax to work with both Python >= 2.6
and Python 3.

test/set-context-property

@@ -33,7 +33,7 @@ for path, properties in modems:
 
 	try:
 		context.SetProperty(sys.argv[2], sys.argv[3])
-	except dbus.DBusException, e:
+	except dbus.DBusException as e:
 		print("Error setting context %s property %s: %s" %\
 				(path, sys.argv[2], str(e)))
 		exit(2)