Error when running `./setup.py develop` with python 3.8

When following the instructions in CONTRIBUTING.rst, I get the following error:

```
Best match: futures 3.3.0
Processing futures-3.3.0.tar.gz
Writing /tmp/easy_install-xljlzw2h/futures-3.3.0/setup.cfg
Running futures-3.3.0/setup.py -q bdist_egg --dist-dir /tmp/easy_install-xljlzw2h/futures-3.3.0/egg-dist-tmp-qgujc4yd
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']}
```

Start with CONTRIBUTING.rst and the setup.py develop path, then inspect how the futures dependency is declared for Python 3.8. Reproduce the command in the issue and verify that the setup process no longer attempts to install the Python 2-only futures package.

Written by the indexing model from the issue text.