PYTHON setup.py build_ext --inplace
PYTHON -c "import foo"
PYTHON -c "import a"

######## setup.py ########

from Cython.Build import cythonize
from distutils.core import setup

setup(
  ext_modules = cythonize("*.pyx"),
)

######## foo.pxd ########

cdef void bar()

######## foo.pyx ########

cdef extern from "bar_impl.c":
    void bar()

######## bar_impl.c ########

static void bar() {}

######## a.pyx ########

from foo cimport bar