Users can still set GCLIENT_PY3=0 to use Python 2.

R=ehmaldonado@chromium.org

Change-Id: Ic7657a091bfe3b9c426ab36063b106aadf2d1c84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2161266
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>