ENH: Move `scipy.special` Cython kernels to XSF

[fbourgey]

Several Cython kernels still live in scipy/special/ and look suitable for migration to XSF.

• _convex_analysis.pxd ENH: add convex_analysis functions to XSF #171
• _agm.pxd ENH: add agm to XSF #172
• _boxcox.pxd ENH: Add boxcox to XSF #178
• _ndtri_exp.pxd ENH: Add ndtri_exp to XSF #180
• _hypergeometric.pxd
• _hyp0f1.pxd
• _spence.pxd

Suggested approach:

1. Add header-only implementations and focused tests in XSF.
2. Update SciPy to route the corresponding ufunc kernels through XSF and remove old code in a follow up SciPy PR.

[dschmitz89]

Are these all of scipy.special's cython kernels? If not we could open a tracking issue. Getting rid of cython kernels would be beneficial for binary size and avoid fragment lation.

[fbourgey]

Are these all of scipy.special's cython kernels? If not we could open a tracking issue. Getting rid of cython kernels would be beneficial for binary size and avoid fragment lation.

+1. Let me update the list with only Cython kernels. Feel free to add more.