Fix test to not require copying the astropy_helpers package; fixed a misuse of pkgutil.get_importer in the build_sphinx command that *happened* to work because we were adding a sys.path entry named 'astropy_helpers'.  But really as written this was assuming astropy_helpers was a module name, not a sys.path entry.  pkgutil.get_loader returns the loader for a specific module.
Merge pull request #1 from embray/fix-six

Fix test to not require copying the astropy_helpers package