Add Python3 classifier
Add Python3 classifier

Motivated by caniusepython3[0] reporting curtsies doesn't support Python3.

```shell
$ caniusepython3 --projects curtsies
Finding and checking dependencies ...

You need 1 project to transition to Python 3.
Of that 1 project, 1 has no direct dependencies blocking its transition:

  curtsies
```

0: https://pypi.python.org/pypi/caniusepython3

sebastinas/curtsies,thomasballinger/curtsies