Remove Support for Py2

[dharvey-consbio]

Taking six out of the picture, and removing Py2 compatible code will allow future updates to more fully leverage the advantages of Python 3.

[nikmolnar]

We need to keep Py2 support for a while longer. Restle is used by python-databasin, which in turn is used in an ArcGIS 10.x environment, which is Py2-only.

[dharvey-consbio]

Can't the applications that depend on Py2 continue to use the current version? The non-Py2 version would be for all other applications going forward.

[nikmolnar]

Can't the applications that depend on Py2 continue to use the current version?

They could unless they need a new feature or bug fix, etc. I agree with dropping support eventually, but I don't see any rush to do so.