Import *.pyd file which has another imported file from build-lib

Question

I'm trying to cythonize the python files in the mentioned folder structure to my_build and import the pyd file in test.py.

Folder structure:

|main_folder
|__,setup.py
|__,test.py
|__,src
|____,sample.py
|____sub_folder
     |__,fibo.py

fibo.py

def fib(n):
    a, b = 0, 1
    while b < n:
        print(b)
        a, b = b, a + b

sample.py

import sub_folder.fibo as fibo

class sample:
    def hello():
        print("Hello")
        fibo.fib(2000)

setup.py

from setuptools import setup, Extension
from Cython.Build import cythonize

ext_modules = extensions = [
    Extension("sub_folder.fibo", ["sub_folder/fibo.py"]),
    Extension("sample", ["sample.py"])
]

setup(
    ext_modules = cythonize(ext_modules, 
        build_dir="build_cfiles")
)

test.py

import my_build.sample as sa

sa.sample.hello()

The command for building:

python setup.py build_ext --build-lib my_build

Error:

...
import my_build.sample
  File "sample.py", line 2, in init sample
ModuleNotFoundError: No module named 'sub_folder'

This error is coming while executing the test.py file. So how fibo.py can be imported to get the proper result?