pysynphot --> stsynphot transition

[jluastro]

Sometime soon, we will need to migrate SPISEA from ASTROLIB pysynphot to synphot + stsynphot (the Astropy versions). There will no longer be development on pysynphot (as of mid 2023). I don’t think there is anything new that we need in synphot/stsynphot; but various bug fixes and python package support issues will likely start to creep in. If we use stsynphot, the transition should be fairly straight forward.

[wei-lingfeng]

Adding to this: pysynphot doesn't work with the latest numpy anymore, e.g.:
AttributeError: np.alltrue was removed in the NumPy 2.0 release. Use np.all instead.