Migrate to python 3

Given that on most systems Python is still linked to the Python 2.7,
switch explicitly to using Python 3.

Python 2.7 was discontinued on January 1, 2020: [1].

Moreover, there is PEP recommending to use python3 in the shebang: [2].

[1] https://www.python.org/doc/sunset-python-2
[2] https://www.python.org/dev/peps/pep-0394

Change-Id: Ia4ca09e65a451868de738d3dba6f62fdd7c5e50d
diff --git a/tools/util_test.py b/tools/util_test.py
index 1a389f5..ab1133b2 100644
--- a/tools/util_test.py
+++ b/tools/util_test.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (C) 2013 The Android Open Source Project
 #
 # Licensed under the Apache License, Version 2.0 (the "License");