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/+/2907155Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>