I tried importing the built-in supernova model 'hsiao' following the instructions on the SNCosmo website under the "Supernova Models" section. In Python 3, the line

model = sncosmo.Model(source='hsiao')

breaks with the error TypeError: can't use a string pattern on a bytes-like object. The error comes from this line in builtins.py:

urls = json.load(f)

which fails because the previous line,

f = urlopen("http://sncosmo.github.io/data/urls.json")

returns raw bytes in Python 3, whereas in Python 2 it returns a string (which is what json.load() expects).

This error does not occur in either version of Python if the requested SN model has already been downloaded and cached.