Revert "cros: switch CrOS programs to python3"

This reverts commit 35b70cac56.

Reason for revert: causing problems for the chromium cq, e.g.
https://ci.chromium.org/p/chromium/builders/try/chromeos-amd64-generic-rel/471366
https://ci.chromium.org/p/chromium/builders/try/chromeos-arm-generic-rel/197740

Original change's description:
> cros: switch CrOS programs to python3
> 
> We start warning if people have outdated systems.
> 
> Bug: 997354
> Test: `cros --help` uses python3 now
> Change-Id: I1491f1c0eea4d544cc508b461153c54c8072699d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2052243
> Commit-Queue: Mike Frysinger <vapier@chromium.org>
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

TBR=vapier@chromium.org,iannucci@chromium.org,saklein@chromium.org,infra-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: Ie85691d9f847ad7edb4c9932a0788431c886fec2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 997354
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2052543
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>

cros

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 # Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -16,13 +16,6 @@ import os
 import sys
 
 
-# Min version of Python that we *want*.  We warn for older versions.
-MIN_PYTHON_VER_SOFT = (3, 6)
-# Min version of Python that we *require*.  We abort for older versions.
-# TODO(vapier): Hard require Python 3.6 by end of 2020.
-MIN_PYTHON_VER_HARD = (3, 4)
-
-
 def _FindChromite(path):
   """Find the chromite dir in a repo, gclient, or submodule checkout."""
   path = os.path.abspath(path)
@@ -53,27 +46,7 @@ and retry.  If you need to setup a Chromium OS source tree, see
   return 127
 
 
-def _CheckPythonVersion():
-  """Verify active Python is new enough."""
-  if sys.version_info >= MIN_PYTHON_VER_SOFT:
-    return
-
-  progname = os.path.basename(sys.argv[0])
-  print('%s: Chrome OS requires Python-%s+, but found "%s"' %
-        (progname, '.'.join(str(x) for x in MIN_PYTHON_VER_SOFT),
-         sys.version.replace('\n', ' ')), file=sys.stderr)
-  if sys.version_info < MIN_PYTHON_VER_HARD:
-    print('%s: fatal: giving up since Python is too old.' % (progname,),
-          file=sys.stderr)
-    sys.exit(1)
-
-  print('warning: temporarily continuing anyways; you must upgrade soon to '
-        'maintain support.', file=sys.stderr)
-
-
 def main():
-  _CheckPythonVersion()
-
   chromite_dir = _FindChromite(os.getcwd())
   target = os.path.basename(sys.argv[0])
   if chromite_dir is None: