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>