Indicate support for Python 3

[Flimm]

This is useful for certain sites like Django Packages. That way, it easy to programmatically detect whether the package is meant to support Python 3. For example, it would be nice if this link indicated that django-smart-selects supported Python 3:

https://djangopackages.org/grids/g/auto-complete/

I'm assuming that this package does indeed support Python 3. If not, this classifier should be used instead: "Programming Language :: Python :: 2 :: Only"

[coveralls]

Coverage remained the same at 89.809% when pulling 446bc8e on Flimm:patch-1 into d2b8772 on digi604:master.

[coveralls]

Coverage remained the same at 89.809% when pulling 446bc8e on Flimm:patch-1 into d2b8772 on digi604:master.