Detectron: FYI: Python 3 support is here!

This is an FYI issue to keep people informed of an upcoming change.

In the near future (1-2 weeks baring unexpected issues), we plan to release updates to fully support Python 3. Python 3 will be preferred over Python 2, though both should work and whichever your default #!/usr/bin/env python is will be the one that runs Detectron. We will continue to support both versions for some time going forward, but will eventually phase out Python 2 support in the sense of no longer actively testing that it works, though we will likely address breakage in a reactive manner.