WIP: Preparations for lib3to6 transpiler

[gonzalocasas]

This is a very quick test to see whether lib3to6 could be used to transpile compas code for IronPython 2.7 compatibility.

Nothing in this PR is to be merged at this point, it's really just a quick'n'dirty check.

What type of change is this?

• [x] Bug fix in a backwards-compatible manner.
• [ ] New feature in a backwards-compatible manner.
• [ ] Breaking change: bug fix or new feature that involve incompatible API changes.

Checklist

1. [ ] Add the change to the CHANGELOG.md file in the Unreleased section under the most fitting heading: Added, Changed, Removed.
2. [ ] Run all tests on your computer (i.e. invoke test).
3. [ ] If you add new functions/classes, check that:
   1. [ ] Are available on a second-level import, e.g. compas.datastructures.Mesh.
   2. [ ] Add unit tests (especially important for algorithm implementations).

[tomvanmele]

cool. travis is no liking it though :)

[jf---]

@gonzalocasas lib3to6 seems very promising, is the IronPython interpreter having it? Since, It has its own particularities? This seems a wonderful route towards a type version of compas?