This is a reland of dbca865e

Original change's description:
> depot_tools: Bootstrap Python 3 on Linux/Mac
> 
> This will make it possible for developers to execute depot_tools
> scripts using Python 3 in a known environment.
> 
> Bug: 1002153
> Change-Id: I5ff492a49d227c1b5876f49adba020f51a575bdd
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1762664
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Reviewed-by: Andrii Shyshkalov <tandrii@google.com>

Bug: 1002153
Change-Id: I6938a76ffa05366c21f7c56aed3d07ecb2b84443
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1798845Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>