How to get the same synphot results in Astropy v2:
>>> import numpy as np
>>> from astropy import constants as const
>>> from astropy import units as u
>>> from astropy.modeling import models as _models
>>> from synphot import models
>>> from synphot import units
>>> t = 5500 * u.K
>>> fac = const.sigma_sb * t ** 4 / np.pi