This is a reland of e53a5939

Additional bug fixes:
* handle race condition in gsutil when creating its directory
* limit to one gsutil process on darwin due to bug in python3

Original change's description:
> Reland "Update gsutil to use gsutil version 4.61, python3."
>
> This reverts commit af121aee.
>
> Reason for revert: re-landing with a switch back to vpython to get the compiled C extension version of crcmod for performance.
>
> Original change's description:
> > Revert "Update gsutil to use gsutil version 4.61, python3."
> >
> > This reverts commit f059ec93.
> >
> > Reason for revert: Reverting because we probably need to be using vpython and a compiled crcmod instead. See, e.g.,. b/188591640.
> >
> > Original change's description:
> > > Update gsutil to use gsutil version 4.61, python3.
> > >
> > > This CL updates the gsutil.py wrapper to download and use
> > > v4.61 of GCP's gsutil, which is Python3-compatible.
> > >
> > > v4.61 appears to be fully self-contained and have all of the
> > > packages it needs vendored into it. So, there's no reason to
> > > use vpython anymore, and this CL removes that.
> > >
> > > Also, this CL removes the 'fallback' option to gsutil and
> > > the ability to force a version switch, as this should no
> > > longer be necessary (it was added for a migration back in 2014
> > > but apparently this code was never removed afterwards).
> > >
> > > This CL also updates download_from_google_storage.py and
> > > upload_to_google_storage.py to similarly not have the version flags
> > > and to just use regular python3, not vpython3.
> > >
> > > Bug: 1184108
> > > Change-Id: I0d1a8351dba2d3ad1f927afa333fb10959f19443
> > > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2898439
> > > Reviewed-by: Mike Frysinger <vapier@chromium.org>
> > > Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> > > Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> > > Commit-Queue: Dirk Pranke <dpranke@google.com>
> >
> > Bug: 1184108
> > Change-Id: I8e21a9a40d81e4e185642f866855b6838f80f1c2
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2905904
> > Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
> > Commit-Queue: Dirk Pranke <dpranke@google.com>
>
> Bug: 1184108
> Change-Id: I5d6d6d06842e08517488471c144972818fcf02ff
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2907155
> Reviewed-by: Mike Frysinger <vapier@chromium.org>
> Reviewed-by: Josip Sokcevic <sokcevic@google.com>
> Commit-Queue: Dirk Pranke <dpranke@google.com>

Bug: 1184108
Change-Id: Ibb5d886fd22e3553521ff8ad6e2b4435844ef972
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2988716Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>