Run roll-dep using Python 3 when GCLIENT_PY3 is set to 1.

Bug: 984182
Change-Id: I511b4cf34b4f5522218a24dff6d0ce9ef5c36c4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2080614Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>