Fixes an issue with the build_sphinx test

This is a fix that I accidentally rolled into #110, but I figured it should probably be pulled out as a separate change.

This one is a little hard to explain, but the main issue is that this was copying the entire astropy_helpers source directory into the temp dir, rather than just the `astropy_helpers` package itself.  This included, for example, all my `.tox` directories which can get quite huge, slowing down the test run and filling up `/tmp`.  So this should fix that, I think.  This should  be merged first, then #110 rebased on top of it.