Revert use of environment markers in extras_require

The use of an “environment marker” in the extras_require dict in #1018 is not compatible with earlier versions of Python 2:

python --version
pushd $(mktemp -d) >/dev/null
git clone -q https://github.com/docker/docker-py.git
virtualenv -q pip
virtualenv -q setuptools

source setuptools/bin/activate
pushd ./docker-py >/dev/null
python setup.py install >/dev/null
popd >/dev/null
pip freeze|grep ipaddress

source pip/bin/activate
pip install -qU pip
pushd ./docker-py  >/dev/null
pip install -q .
popd >/dev/null
pip freeze|grep ipaddress
popd >/dev/null

Output on Python 2.7.10

Python 2.7.10
py2-ipaddress==3.4.1
py2-ipaddress==3.4.1

Output on Python 2.7.3

Python 2.7.3
error in docker-py setup command: Invalid environment marker: python_version < "3"
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-XrHLjZ-build/

[shin-]

There is no “fix” planned - this is not a bug. Please update your version of setuptools to support the environment marker syntax.

[shin-]

Can this be fixed by updating your setuptools package? I just tested it with Python 2.7.7 and simply doing pip install -U setuptools seems to solve the problem.