Python 3+ support and MIT license file

[mrakitin]

This PR adds the Python 3+ support (based on master...ronpandolfi:master changes) and adds a missing LICENSE file with the MIT license text.

xref #2 (comment).

[mikapfl]

I tested building, installing, and importing pygix under python 3 with these patches and it works nicely. Unfortunately, I don't have the time to try out a full workflow (calibration, transformation of image, etc.).

Note that I'm no developer of pygix, this is just a drive-by comment to confirm that it works and thank you for your patches. (-:

[mrakitin]

Hi @mikapfl, thanks for giving it a try! I actually created a recipe for conda and submitted it to conda-forge (via conda-forge/staged-recipes#16399), and now we have this feedstock to build the package - https://github.com/conda-forge/pygix-feedstock. Please feel free to add yourself as a maintainer if you wish.

The advantage of conda is that it can perform tests for the packaged code, so I added a number of imports that actually captured the missing pyyaml dependency. I'll add it here too.

[mrakitin]

Thank you, @tgdane!