BUG: Fix build of third-party extensions with Py_LIMITED_API
BUG: Fix build of third-party extensions with Py_LIMITED_API

The type `vectorcallfunc` is not defined by Python's limited API.
Guard its use with a `#ifdef` so that third-party extensions that
use the Numpy C API will build with Py_LIMITED_API defined.

This fixes a regression introduced in #20315.

charris/numpy,seberg/numpy,numpy/numpy,charris/numpy,jakirkham/numpy,rgommers/numpy,mattip/numpy,mattip/numpy,rgommers/numpy,seberg/numpy,seberg/numpy,numpy/numpy,rgommers/numpy,charris/numpy,charris/numpy,anntzer/numpy,endolith/numpy,pdebuyl/numpy,jakirkham/numpy,mattip/numpy,jakirkham/numpy,mhvk/numpy,endolith/numpy,rgommers/numpy,anntzer/numpy,mattip/numpy,anntzer/numpy,numpy/numpy,mhvk/numpy,pdebuyl/numpy,pdebuyl/numpy,numpy/numpy,mhvk/numpy,mhvk/numpy,jakirkham/numpy,anntzer/numpy,pdebuyl/numpy,seberg/numpy,endolith/numpy,jakirkham/numpy,endolith/numpy,mhvk/numpy