Only use the futures backport on Python 2.7
Only use the futures backport on Python 2.7

The `futures` package does not install on Python 3:

```
Collecting futures (from caniusepython3==6.0.0)
  Using cached https://files.pythonhosted.org/packages/4a/f4/418e844d868e34638486732417fb82b05031910059d88b86aaea9c70f699/futures-3.1.2.tar.gz
    Complete output from command python setup.py egg_info:
    This backport is meant only for Python 2.
    It does not work on Python 3, and Python 3 users do not need it as the concurrent.futures package is available in the standard library.
    For projects that work on both Python 2 and 3, the dependency needs to be conditional on the Python version, like so:
    extras_require={':python_version == "2.7"': ['futures']}
```

brettcannon/caniusepython3