errors finding package when using tox for testing with cython extensions

Question

I am using cython to build a module for a library. When I install the package locally, i.e., pip install . I can perfectly import and run the module, however I cannot do that in tox environments.

Locally I get a ModuleNotFoundError: No module named 'pylandstats.adjacency', whereas in GitHub Actions i get a ValueError: 'pylandstats/adjacency.pyx' doesn't match any files (see the latest run with verbose tox).

You can find the tox config at pyproject.toml, cython extension at setup.py and GitHub Actions workflow at .github/workflows/tests.yml here: https://github.com/martibosch/pylandstats/pull/60

How can I fix this? Thanks in advance!