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

This is a reland of commit c0af61c8dd

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>

gsutil.py

@@ -34,7 +34,7 @@ DEFAULT_BIN_DIR = os.path.join(THIS_DIR, 'external_bin', 'gsutil')
 
 IS_WINDOWS = os.name == 'nt'
 
-VERSION = '4.28'
+VERSION = '5.6'
 
 
 class InvalidGsutilError(Exception):
@@ -166,9 +166,7 @@ def run_gsutil(target, args, clean=False):
   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)

gsutil.py.vpython3

@@ -0,0 +1,19 @@
+# vpython VirtualEnv protobuf for "gsutil".
+#
+# See:
+# https://chromium.googlesource.com/infra/luci/luci-go/+/HEAD/vpython/api/vpython/spec.proto
+#
+# This is a "vpython" VirtualEnv specification applied to invocations of "gsutil"
+# by the bootstrap wrapper, "gsutil.py". It ensures that any Python distribution
+# has the expected Python packages installed.
+#
+# This is specifically relevant on Google Compute Engine invocations of
+# "gsutil", where a stock system-deployed file in "/etc/" explicitly specifies
+# that the "google_compute_engine" and "boto" Python packages are available.
+
+python_version: "3.8"
+
+wheel: <
+  name: "infra/python/wheels/crcmod/${vpython_platform}"
+  version: "version:1.7.chromium.3"
+>

gsutil.vpython

@@ -1,104 +0,0 @@
-# vpython VirtualEnv protobuf for "gsutil".
-#
-# See:
-# https://chromium.googlesource.com/infra/luci/luci-go/+/HEAD/vpython/api/vpython/spec.proto
-#
-# This is a "vpython" VirtualEnv specification applied to invocations of "gsutil"
-# by the bootstrap wrapper, "gsutil.py". It ensures that any Python distribution
-# has the expected Python packages installed.
-#
-# This is specifically relevant on Google Compute Engine invocations of
-# "gsutil", where a stock system-deployed file in "/etc/" explicitly specifies
-# that the "google_compute_engine" and "boto" Python packages are available.
-
-python_version: "2.7"
-
-wheel <
-  name: "infra/python/wheels/google_compute_engine-py2_py3"
-  version: "version:2.6.2"
->
-wheel <
-  name: "infra/python/wheels/boto-py2_py3"
-  version: "version:2.48.0"
->
-
-# "gsutil" on non-GCE can require PyOpenSSL, which, in turn, requires
-# "cryptography".
-
-wheel: <
-  name: "infra/python/wheels/pyopenssl-py2_py3"
-  version: "version:17.2.0"
->
-
-##
-# BEGIN "cryptography" dependencies.
-##
-
-wheel: <
-  name: "infra/python/wheels/cryptography/${vpython_platform}"
-  version: "version:2.9.2.chromium.1"
->
-
-wheel: <
-  name: "infra/python/wheels/appdirs-py2_py3"
-  version: "version:1.4.3"
->
-
-wheel: <
-  name: "infra/python/wheels/enum34-py2"
-  version: "version:1.1.6"
->
-
-wheel: <
-  name: "infra/python/wheels/cffi/${vpython_platform}"
-  version: "version:1.14.5.chromium.7"
->
-
-wheel: <
-  name: "infra/python/wheels/idna-py2_py3"
-  version: "version:2.5"
->
-
-wheel: <
-  name: "infra/python/wheels/ipaddress-py2"
-  version: "version:1.0.18"
->
-
-wheel: <
-  name: "infra/python/wheels/packaging-py2_py3"
-  version: "version:16.8"
->
-
-wheel: <
-  name: "infra/python/wheels/pyasn1-py2_py3"
-  version: "version:0.2.3"
->
-
-wheel: <
-  name: "infra/python/wheels/pycparser-py2_py3"
-  version: "version:2.17"
->
-
-wheel: <
-  name: "infra/python/wheels/pyparsing-py2_py3"
-  version: "version:2.2.0"
->
-
-wheel: <
-  name: "infra/python/wheels/setuptools-py2_py3"
-  version: "version:34.3.2"
->
-
-wheel: <
-  name: "infra/python/wheels/six-py2_py3"
-  version: "version:1.10.0"
->
-
-##
-# END "cryptography" dependencies.
-##
-
-wheel: <
-  name: "infra/python/wheels/crcmod/${vpython_platform}"
-  version: "version:1.7.chromium.3"
->