How to make python relative imports of a module work in this simple setup?

Question

I am using Python 3.10.4 and I have the following setup:

__init__.py
a.py
child
  __init__.py
  b.py

a.py:

def add(a,b):
 return a+b

b.py:

 from ..a import add
 print(add(2,3))

init.py files are empty.

My issue is that in this setup running python child/b.py from the top level directory here always results in "ImportError: attempted relative import with no known parent package"

I have tried various suggestions from Relative imports in Python 3 including trying to append the path using the following but that didn't help :

import sys
import os

SCRIPT_DIR = os.path.dirname(os.path.abspath(__file__))
sys.path.append(os.path.dirname(SCRIPT_DIR))

The only way I have been able to make this work is by using an absolute import and exporting the working directory path to PYTHONPATH using export PYTHONPATH=.

How do I make relative imports work?

Thanks!