Stop using NPY_NDARRAYOBJECT_H since:
a) in principle it's private
b) Numpy has renamed it
and use a public symbol instead.

I think the existing tests are adequate - we just aren't yet testing
against a new enough version of Numpy to have caught it yet.

Closes https://github.com/cython/cython/issues/4396
Closes https://github.com/cython/cython/issues/4394