[MAINT] Von-Mises spline

I took a look at why `pyximport` was being used at `setup.py` to calculate spline coefficients which are compiled into one of the c files for the von mises hmm, and made a couple changes.

- The spline data is saved in the repo, so there's not need to actually regenerate it at compile time.
- The code that generates the spline data is mostly just for reference, so that the source of the numbers isn't a mystery. But that code doesn't need to be cython. It can just be pure python.
- We can solve the tridiagonal system that's required to get the spline coefficients without a custom solver, and using scipy instead.

So I made these changes and regenered the spline data. You can see in the diff that the numbers change in approximately the 12th decimal point.