relative import in test directory

Question

My typical Python project structure looks like

setup.py
foobar/
    __init__.py
    main.py
    something_else.py
    helpers.py
test/
    addition/
        test_consistency.py
        test_speed.py
    subtraction/
        test_consistency.py
        test_speed.py

This works well.

I then notice that the addition and subtraction tests share quite a bit of code, complex assertions and such. I tried to move the code into a helper file

test/
    helpers.py
    addition/
        test_consistency.py
        test_speed.py
    subtraction/
        test_consistency.py
        test_speed.py

and import from addition/subtraction like

from .. import helpers 

but this fails with

E   ImportError: attempted relative import with no known parent package

Alright, the test folder is not a "package" and relative imports don't work. A well-known Python issue.

How can I share code between the addition and subtraction subfolders? A flat directory structure is not an option. (In real life, the folder structure is more complex than the above example.)

Weirdly, I noticed that pyscaffold does to relative imports in tests/, see here. How do they do that?