Update run_on_app to be python3

Change-Id: I770818b21cd90dd7a553186f7cf6d1ba2ef6260c
diff --git a/tools/run_on_app.py b/tools/run_on_app.py
index 983ce6d..14310b6 100755
--- a/tools/run_on_app.py
+++ b/tools/run_on_app.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (c) 2017, the R8 project authors. Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.