Don't import IPython

I'm investigating various things that make importing astropy slow.

One thing we're doing here is importing `IPython` unconditionally -- which is fairly expensive, as IPython is a large project.  I think we did that at one time to change console behavior depending on the IPython context, but since changed it to using the `get_ipython()` trick, which only has effect if the user is already running inside of IPython.