update to use pystan>=3?

[justindomke]

It's becoming increasingly difficult to install this package due to reliance on pystan v2. This seems to cause a chain of requirements of other out of date packages.

Could we update it to use pystan v3 instead?

I think the changes that are needed would be

1. In setup.py (on pypi) chance pystan==2.whatever to pystan>=3
2. In interface.py change import pystan to import stan
3. In interface.py update the code to use the newer version of pystan signatures.

Incidentally, I looked into using bridgestan and I don't suggest we convert to it. The big advantage of pystan is that it actually seems to be able to install stan, from scratch, with just a call to pip. Bridgestan requries a lot of manual configuration of stuff.