Cython is installed but doesn't seem to do anything

I can successfully execute `%%load_ext cython` and then run a cell like this:

```
%%cython -a --verbose

def test():
return 5 + 6
```

This outputs:

```
[1/1] Cythonizing /var/mobile/Containers/Data/Application//Documents/.ipython/cython/_cython_magic_92a929e066e52d7b4b226cb8d367b669af63f47f.pyx
building '_cython_magic_92a929e066e52d7b4b226cb8d367b669af63f47f' extension
```

...and then it completes without any errors, but calling `test()` keeps raising `NameError`.

I assume this is because there's no C compiler, like in https://github.com/holzschu/Carnets/issues/125, but why have Cython if there's no C compiler? Am I missing something?

---------

Carnets version: 1.5.5

Reproduce the notebook using the `%%load_ext cython` and `%%cython -a --verbose` entry points from the report, then inspect why the generated extension does not make `test()` available. Done means either the cell-defined function can be called successfully or the unsupported compiler behavior is clearly surfaced to users.

Written by the indexing model from the issue text.