roll-dep: Make it possible to run using Python 3.
roll-dep: Make it possible to run using Python 3.

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/+/2080614
Reviewed-by: Anthony Polito <05bb588aa9020353543fee7ac4e6e0a3d0d6f6ed@google.com>
Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools