setuptools v36.0.0 breaks the python plugin

[Kyle Fazzari]

[Impact]

 * setuptools v36.0.0 was released today, which no longer bundles six, thereby breaking the python plugin (both python2 and python3).
 * The fix is to simply install six before setuptools.

[Test Case]

 * Simply run one of the many python-based integration tests. For example:

       ./runtests.sh plugins test_python_plugin.py

   Without this patch, the test will fail. With it, the test will pass.

[Regression Potential]

 * Potential is low as we have a large test suite with a number of python-based snaps.
 * Pulling in six could potentially conflict with the dependencies of the snap being build, although six was already used previously (it was just bundled with setuptools).

[Kyle Fazzari (kyrofa)]

Fix: https://github.com/snapcore/snapcraft/pull/1339

[Steve Langasek (vorlon)]

An upload of snapcraft to zesty-proposed has been rejected from the upload queue for the following reason: "change not needed, fixed upstream in setuptools instead".