Document Python 3 compatibility

It's not clear from the `README.md` file whether Python 3 is supported or not.

Also, the `setup.py` file should indicate whether it supports Python 3 like this:

```
'Programming Language :: Python :: 3',
```

Thanks!
##

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/31682680-document-python-3-compatibility?utm_campaign=plugin&utm_content=tracker%2F332251&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F332251&utm_medium=issues&utm_source=github).

Start with README.md and setup.py, which are the files named in the issue. Confirm the project's Python 3 support, document it in the README, and add the Python 3 programming-language classifier to setup.py. Done means both files clearly indicate Python 3 compatibility.

Written by the indexing model from the issue text.