Move to Python 3

[vikhari]

Python 2 was used for Ontospy and other tools. If Python 3 can be used for Autogeneration and the API itself, etc. then we should transition. This should likely be done after reaching a stable version of CASE to first get the workflow and framework more stable.

[vikhari]

Python 3 may be able to use other (simpler) ways to type check, possibly dynamically, which could be used to change the API. I have not looked into these yet, but here are some links for future reference:

• https://github.com/facebook/pyre-check
• https://www.python.org/dev/peps/pep-0484/
• http://mypy-lang.org/