add astropy and sbypy to requirements

[mschwamb]

add astropy and sbypy to requirements both for docs (for future demo notebooks) and for pip installation

Fixes #45

• Does pip install still work?

[jrob93]

Thanks for adding these Meg. Do we also want to add matplotlib for making plots in demo notebooks?

[mschwamb]

That would only need to go into the docs requirements if only the jupyter notebooks demos are going to have plots. My suggestion is if you think the main package will require matplotlib then you open a pull request and follow what I did in this PR to add it. A good way of learning how to add requirements.

[mschwamb]

So I would approve this and add in a new issue for matplotlib and make a separate PR for that