Migrate to Python 3

[boulund]

The project should be migrated to Python 3 as soon as Gooey implements official support for wxPython 4.0.0a1 (Phoenix; the first version of wxPython with Python 3.4+ support).

This is a low priority issue, but will bring benefits in the future as we will only need a single Python 3.6+ environment instead of maintaining both 2.7 and 3.6+ environments on our machines for running our own scripts.