None of the `cython_*` APIs have any tests using Cython

There are Python-level tests, e.g. scipy/linalg/tests/test_cython_blas.py. However, no tests failed when some .pxd files went missing in the Meson build. We need some basic tests that cimport some functionality and invoke Cython in a subprocess. Submodules that need testing:

• linalg.cython_blas
• linalg.cython_lapack
• optimize.cython_optimize
• special.cython_special

Test infrastructure can be stolen from https://github.com/numpy/numpy/blob/main/numpy/random/tests/test_extending.py

[Kai-Striega]

Ok great, I’ll get to it over the Christmas break

[rgommers]

I’d say yes, that is enough for now, one or two functions per cython module. The tests I linked in the PR description builds an extension with cython (https://github.com/numpy/numpy/tree/main/numpy/random/_examples/cython), and then not only imports it but uses the API. If that works for one function in a module, it’s quite likely that it works for all of them. So while complete coverage would be nice, that’s a large task that’s best left until we see a need.