Migrate build/ to Python 3

Test: mm
Bug: 200119288
Change-Id: I0594074b9a74ec7272da325232e7bd8ec0ec705b

build/Android.bp

@@ -31,12 +31,4 @@ python_binary_host {
     "secilc",
     "version_policy",
   ],
-  version: {
-    py2: {
-      enabled: true,
-    },
-    py3: {
-      enabled: false,
-    },
-  },
 }

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");

build/file_utils.py

@@ -39,7 +39,7 @@ def filter_out(pattern_files, input_file):
         patterns.extend(open(f).readlines())
 
     # Copy lines that are not in the pattern.
-    tmp_output = tempfile.NamedTemporaryFile()
+    tmp_output = tempfile.NamedTemporaryFile(mode='w+')
     with open(input_file, 'r') as in_file:
         tmp_output.writelines(line for line in in_file.readlines()
                               if line not in patterns)