ROS Cython: dynamic module does not define module export function (PyInit_...)

I have made a python wrapping of a c++ class similar to here and here. but the generated python wrapper is only usable in python2 while giving the following error in python3:

dynamic module does not define module export function (PyInit_...)

my setup.py looks like the following:

## ! DO NOT MANUALLY INVOKE THIS setup.py, USE CATKIN INSTEAD

from distutils.core import setup
from catkin_pkg.python_setup import generate_distutils_setup

setup_args = generate_distutils_setup(
    packages=['hw_interface_pkg'],
    package_dir={'': 'src'},
    requires=['std_msgs', 'rospy'],
    language_level="3",
    compiler_directives={'language_level' : "3"}
)

setup(**setup_args)

following this thread I tried adding language_level = "3" or compiler_directives={'language_level' : "3"} to the above setup_args but none solved the problem.

my pyx file also looks like:

cdef extern from "remote_hw.h":
    cdef cppclass ROBOTHardwareInterface:
        ROBOTHardwareInterface() except +
        double c_fibonacci(double n);

cdef class PyHWInterface:
    cdef ROBOTHardwareInterface c_obj
    def __cinit__(self):
        pass
    def fibonnaci_func(self,n):
        return self.c_obj.c_fibonacci(n)

-------------edit

I made sure setup.py is called by python3. but still the same issue.....