Make code compatible with both Python 2.7 and Python 3.6

[stennie]

Python 2.7 will eventually sunset in 2020.

It looks like mtools doesn't have any dependencies that haven't been ported to Python 3 yet, so we should consider making the code compatible with both Python 2.7 and a modern version of 3.x (perhaps 3.6 to limit the test surface?):

• https://docs.python.org/3/howto/pyporting.html
• http://python-future.org/compatible_idioms.html
• http://python3porting.com