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/blockdiff.py b/scripts/blockdiff.py
index 5793def..95893cf 100755
--- a/scripts/blockdiff.py
+++ b/scripts/blockdiff.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python2
+#!/usr/bin/env python
 #
 # Copyright (C) 2013 The Android Open Source Project
 #
@@ -17,6 +17,7 @@
 
 """Block diff utility."""
 
+from __future__ import absolute_import
 from __future__ import print_function
 
 # pylint: disable=import-error
@@ -46,7 +47,7 @@
 
   """
   if max_length < 0:
-    max_length = sys.maxint
+    max_length = sys.maxsize
   diff_list = []
   num_blocks = extent_start = extent_length = 0
   while max_length or extent_length: