Drop Python 2.7 support

[omry]

I am planning to drop support for Python 2.7 from OmegaConf (issue) and Hydra in the next major release.

Python 2.7 is reaching it's end of life in 1/1/2020, and Hydra supporting both Python 2.7 and Python 3.x is hard. In addition it prevents usage of modern Python features like typed Python.

General info on Python 2.7 here.

For people that will need to keep using Hydra with unsupported Python versions, the 0.11 release will still be available but new features will not support Python 2.7 and will not be added to it.