have setup.py inspect the python version to determine dependencies

Summary:
Our Arch package is broken because it doesn't install asyncio and
pathlib. These shouldn't *need* to be installed, because Arch is running
Python 3.4, but the executable script that setup.py generates is getting
confused. It seems like the right thing to do is to make sure these
extra dependencies never make it into our `install_requires` list in
Python 3.4. I *think* the fact that our setup.py ends up in the source
distribution means that `pip install peru` will still do the right thing
for Python 3.3.

Test Plan: Tested `pip install .` in a python3.3 virtualenv.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D151