Import fails on Python 3 if setup.py exists in current directory

The `__init__.py` test of whether the user is in the astropy source directory just checks for the existence of `setup.py`.  This can easily have false positives, as first noted in #1875.
```
    if sys.version_info[0] >= 3 and os.path.exists('setup.py'):
        _rollback_import(
            "You appear to be trying to import astropy from within a source "
            "checkout. This is currently not possible using Python 3 due to "
            "the reliance of 2to3 to convert some of Astropy's subpackages "
            "for Python 3 compatibility.")
```