Remove dependency on six and enable sphinx tests on Python 3

This all shouldn't be necessary, in principle.  All the other tests are set up so that the astropy_helpers is directly importable from the source checkout when running the setup.py's for the test packages.  This is especially advantageous because it means the coverage tracer can pick up parts of astropy_helpers that are covered by the test even when used via setup.py.  This is why it goes to the pain of using the `run_setup` function from `setuptools.sandbox` (which runs setup.py in the same process) rather than running it in a subprocess, in most cases.