Issue with non-cross-platform wheels generated by setup.py

Hello.

We have got an issue with the Python package building and usage in a cross-platform scenario.

Running `python setup.py bdist_wheel` generates a `prophet-1.0.1-py3-none-any.whl`, a wheel containing the package one can install and distribute.

The produced whl file is marked as "pure", i.e. platform-independent (as says "any" suffix). This implies that, once uploaded to a Python package repository, it can be installed and used on any platform (any OS/CPU).

However, as far as I understand, the wheel contains a pickle file with a compiled STAN model. The compiled model is not cross-platform by itself and so is the wheel then.

Namely, a wheel built on mac can be installed on a linux machine with `pip install`. However, when using the package, we run into an exception preceded by the following warning:
> [2021-07-30T15:13:38.400Z] WARNING pystan:model.py:438 Something went wrong while unpickling the StanModel. Consider recompiling.

This becomes a real issue, for example, when a local Python repository is used to distribute prebuilt packages across machines with different OS.

We would like `setup.py` being able to build a wheel tagged properly according to the platform it is built on, with PEP425 platform tag instead of "any", e.g.
* prophet-1.0.1-py3-none-linux_x86_64.whl
* prophet-1.0.1-py3-none-macosx_11_0_x86_64.whl
etc.

We can submit a PR to fix this. Please let me know if this is of interest.

Thanks,
Max