Add Python 3 support

[mjnowen]

http://www.vfxplatform.com/

29th April 2017 - CY2018 Draft published with significant upgrades to gcc and glibc. This also comes with notice of our intention to move to Python 3 for CY2019 in advance of Python 2 support ending in 2020. We are currently soliciting feedback on this Draft so please either send to feedback@vfxplatform.com or share on vfx-platform-discuss. We expect to publish CY2018 Final around SIGGRAPH in July.