sasview - all non-gui tests should be converted to run in Python 3

[andyfaff]

As part of the progress to upgrade sasview to be able to use Python 3 we should be able to run all non-gui code of sasview in Python 3.
To start with this means running files through the 2to3 tool, and will address things such as:

• print being a function
• raising exception objects, i.e. raise ValueError?(msg) instead of raise ValueError?, msg.
• etc.

The CI testing should be using python 3 as well.

[krzywon]

PR109 added py3 support to sasview while maintaining py2 compatibility. All unit tests now pass when running py2 or py3.