Make gtest-parallel to use python3

diff --git a/gtest-parallel b/gtest-parallel
index c94b082..944f4fc 100755
--- a/gtest-parallel
+++ b/gtest-parallel
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright 2017 Google Inc. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");