How to structure a Python3 project for running a web service and running a test suite (import issues)

Question

I have a simple Python web service built around FastAPI that looks like this

project/
  app/
    __init__.py
    main.py
    config.py
    tests/
      run_tests.py

main.py needs to import from config.py some items, it looks like this

from app.config import Settings

app = FastAPI()

and I need to run it like this to run the webservice

 $ cd project/app
 $ python main.py
 $ # Or I can do this
 $ uvicorn main:app --reload

run_tests.py has this code

from app.main import app
from app.config import settings

and I run the tests like this

$ cd project/app/tests
$ pytest

---

However, this doesn't work. I can only ever get either the tests to work or I can get the webserver to work.

1. If I leave the code as it is above the unit tests run fine but I can't run the webservice

$ cd project/app
$ python main.py
Traceback (most recent call last):
  File "main.py", line 19, in <module>
    from app.config import settings
ModuleNotFoundError: No module named 'app'
$ cd ../../project 
$ # I'm only in '/project' now not 'project/app'
$ python app/main.py
Traceback (most recent call last):
  File "app/main.py", line 19, in <module>
    from app.config import settings
ModuleNotFoundError: No module named 'app'

If I try to change it a relative import

main.py

from .config import Settings

I get this error

$ cd project/app
$ python main.py
Traceback (most recent call last):
  File "main.py", line 19, in <module>
    from .config import settings
ModuleNotFoundError: No module named '__main__.config'; '__main__' is not a package

It only works if I change it as follows

main.py

from config import Settings

$ cd project/app
$ python main.py
$ python main.py
INFO:     Started server process [8325]
INFO:uvicorn.error:Started server process [8325]
INFO:     Waiting for application startup.

However - this breaks the running of the unit tests

main.py

from config import Settings

$ cd project/app/tests
$ pytest
test_api.py:7: in <module>
    from app.main import app
../main.py:19: in <module>
    from config import settings
E   ModuleNotFoundError: No module named 'config'

---

I don't understand what is going on? I have an __init__.py inside of my project/app folder so why isn't that considered a package?