Update remaining scripts to invoke python3.

Change-Id: I72c4058d0381e181a0cade5a4a31b88992ddaa38
diff --git a/tools/dex2oat.py b/tools/dex2oat.py
index e21dc7d..7c5a31e 100755
--- a/tools/dex2oat.py
+++ b/tools/dex2oat.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (c) 2018, 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.