The package runs doctests from setuptools_rust/*.py but not actual tests from
tests/, because pybuild.testfiles exists but doesn't include the tests dir.
Making those tests pass needs more work though: "examples/namespace_package/*"
in pybuild.testfiles needs to be replaced with "examples", B-D on cargo and
librust-pyo3-0.25-dev need to be added, and then something needs to be done so
that cargo find the pyo3 dep (I stopped here due to lack of required
knowledge).