Delete reference to caught exception ``e`` inside try/except block, because on Python 3, it gets deleted at the end of the try/except block, and ``del e`` then fails because ``e`` does not exist anymore. In the core package, this is a bug that would never normally occur, because one cannot import astropy from the source tree in Python 3 anyway (since the code has to be 2to3'd). But the same code causes issues in affiliated packages that have Python 3-compatible code that doesn't need 2to3.
Merge pull request #718 from astrofrog/setup/fix-del-e

Fix to removing reference to caught exception