PYTHON setup.py build_ext --inplace
PYTHON -c "import modb; modb.ClassB()"

#################### moda.pyx ####################

cdef class ClassA:
    cdef int[2] a

#################### modb.pyx #####################

from moda cimport ClassA

cdef class ClassB(ClassA):
    cdef int[2] b

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

from setuptools import setup
from Cython.Build import cythonize
import Cython.Compiler.Options

Cython.Compiler.Options.cimport_from_pyx = True

setup(
        ext_modules = cythonize(["moda.pyx", "modb.pyx"],
            compiler_directives={'language_level': 3})
)