update_payload: Port scripts to python3

Update the update_payload scripts to be compatible with
python2 and python3. Python2 compatibility is needed since
the repo is shared with Android.

BUG=chromium:1011631

TEST=Executed aosp/system/update_engine/scripts/run_unittests and
cros_generate_update_payload

Cq-Depend: chromium:1904837, chromium:1911499
Change-Id: Ie450b80b5f7550051b38d320173ccc0c915f65e7
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/1904310
Commit-Queue: Andrew Lassalle <andrewlassalle@chromium.org>
Tested-by: Andrew Lassalle <andrewlassalle@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Auto-Submit: Andrew Lassalle <andrewlassalle@chromium.org>
diff --git a/scripts/update_device.py b/scripts/update_device.py
index 5c19b89..f970bd3 100755
--- a/scripts/update_device.py
+++ b/scripts/update_device.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/env python
 #
 # Copyright (C) 2017 The Android Open Source Project
 #
@@ -17,8 +17,9 @@
 
 """Send an A/B update to an Android device over adb."""
 
+from __future__ import absolute_import
+
 import argparse
-import BaseHTTPServer
 import hashlib
 import logging
 import os
@@ -29,6 +30,8 @@
 import xml.etree.ElementTree
 import zipfile
 
+from six.moves import BaseHTTPServer
+
 import update_payload.payload
 
 
@@ -41,6 +44,7 @@
 # The port on the device that update_engine should connect to.
 DEVICE_PORT = 1234
 
+
 def CopyFileObjLength(fsrc, fdst, buffer_size=128 * 1024, copy_length=None):
   """Copy from a file object to another.
 
@@ -130,7 +134,6 @@
           start_range = file_size - int(e)
     return start_range, end_range
 
-
   def do_GET(self):  # pylint: disable=invalid-name
     """Reply with the requested payload file."""
     if self.path != '/payload':
@@ -173,7 +176,6 @@
     f.seek(serving_start + start_range)
     CopyFileObjLength(f, self.wfile, copy_length=end_range - start_range)
 
-
   def do_POST(self):  # pylint: disable=invalid-name
     """Reply with the omaha response xml."""
     if self.path != '/update':
@@ -442,5 +444,6 @@
 
   return 0
 
+
 if __name__ == '__main__':
   sys.exit(main())