Migrate build/ to Python 3

Test: mm
Bug: 200119288
Change-Id: I0594074b9a74ec7272da325232e7bd8ec0ec705b
diff --git a/build/build_sepolicy.py b/build/build_sepolicy.py
old mode 100644
new mode 100755
index 285bfea..ce0548a
--- a/build/build_sepolicy.py
+++ b/build/build_sepolicy.py
@@ -1,3 +1,5 @@
+#!/usr/bin/env python3
+#
 # Copyright 2018 - The Android Open Source Project
 #
 # Licensed under the Apache License, Version 2.0 (the "License");