Enable importing Astropy from a source checkout on Python 3

I could have sworn I already had a PR for this but apparently not...

This removes a check in `astropy.__init__` which prevented importing astropy from a git repository when using Python 3.  Previously this was necessary because astropy only worked on Python 3 using 2to3.  But now (with a few small tweaks in this PR) all modules in Astropy work in Python 3 without translation through 2to3.

I don't think Travis-CI will test this, but I confirmed that with this PR I could run

```
$ python3.x setup.py build_ext --inplace
$ python3.x -c 'import astropy; astropy.test()'
```

and that the tests would use the astropy in the source checkout and pass.  I did this with python 3.3 and 3.4.