Improve gclient Python 3 compatibility

This enables gclient sync and gclient runhooks to run, barring hook script failures.
git cl upload also now works.

The scripts still work with Python 2.
There are no intended behaviour changes.

Bug: 942522
Change-Id: I2ac587b5f803ba7f5bb5e412337ce049f4b1a741
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1524583
Commit-Queue: Raul Tambre <raul@tambre.ee>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

detect_host_arch.py

@@ -5,6 +5,8 @@
 
 """Outputs host CPU architecture in format recognized by gyp."""
 
+from __future__ import print_function
+
 import platform
 import re
 import sys
@@ -51,4 +53,4 @@ def DoMain(_):
   return HostArch()
 
 if __name__ == '__main__':
-  print DoMain([])
+  print(DoMain([]))