Reland "Update gsutil to use gsutil version 5.6, python3"

This is a reland of commit c0af61c8dd0fee6a483aacbf64a57d46ba80355e

Original change's description:
> Update gsutil to use gsutil version 5.6, python3
>
> R=gavinmak@google.com
>
> Bug: 1184108
> Change-Id: I933c07132006b731a97153080ebe86ed756fbdb2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3497998
> Reviewed-by: Gavin Mak <gavinmak@google.com>
> Commit-Queue: Josip Sokcevic <sokcevic@google.com>

Bug: 1184108
Change-Id: I72c5b967f52e8b4531c238b20444596c549a5726
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3502172
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
diff --git a/gsutil.py b/gsutil.py
index 61aaafe..90265cf 100755
--- a/gsutil.py
+++ b/gsutil.py
@@ -34,7 +34,7 @@
 
 IS_WINDOWS = os.name == 'nt'
 
-VERSION = '4.28'
+VERSION = '5.6'
 
 
 class InvalidGsutilError(Exception):
@@ -166,9 +166,7 @@
   assert sys.platform != 'cygwin'
 
   cmd = [
-      'vpython',
-      '-vpython-spec', os.path.join(THIS_DIR, 'gsutil.vpython'),
-      '--',
+      sys.executable,
       gsutil_bin
   ] + args_opt + args
   return subprocess.call(cmd, shell=IS_WINDOWS)