Run CI in python3 as well

[Achllle]

Run dual_quaternions tests with Python3 in Travis CI. This requires a few modifications:

1. In setup.py, from distutils.core import setup --> from setuptools import setup although this old article says it doesn't like it but I think that can be disregarded.
2. Code needs to become python3 compatible. Just need to fix one test!
3. Modify Travis config to run python3. Not sure how that would work with ROS installation.

[mjsobrep]

on 3: you might just want to have a seperate job so that it tries it with clean python 3, assuming ROS isn't really needed. Then you can have tests that isolate the different use cases.

Also: just started using github actions, pretty cool

[Achllle]

That's the plan. I saw github actions, might try that for the next project!