The bridge code between https://github.com/sherpa/sherpa and AstroPy I mentioned yesterday is https://github.com/astropy/saba

It's currently set up to provide access to Sherpa's statistics and optimisers to AstroPy modelling, which could be interesting here, but the idea is to also provide access to Sherpa's models from AstroPy (but this is less developed).

astropy.modeling can do chi-square statistics just fine. However, for cases with few counts per bin, the user should have the option to use the C-statistic, which does not appear to be part of astropy.modeling. This should be a set of common routines for the cases with and without background, similarly to how it is implemented in Xspec.