Can't use relative imports from conftest.py in test package with --import-mode=importlib

[mje-nz]

With tests outside application code in a tests package containing conftest.py, pytest --import-mode=importlib doesn't import conftest as part of the package.

Example

Files:

test/__init__.py
test/util.py:
    def helper_function():
        pass
test/conftest.py:
    import pytest
    from .util import helper_function
    
    @pytest.fixture
    def common_fixture():
        helper_function()
test/test_example.py:
    def test_function(common_fixture):
        pass

Repro:

$ pytest                        
================================= test session starts =================================
platform darwin -- Python 3.7.3, pytest-6.0.1, py-1.9.0, pluggy-0.13.1
rootdir: mwe
collected 1 item                                                                                                                                                                    

test/test_example.py .                                                                                                                                                        [100%]

================================== 1 passed in 0.01s ==================================

$ pytest --import-mode=importlib
ImportError while loading conftest 'mwe/test/conftest.py'.
test/conftest.py:3: in <module>
    from .util import helper_function
E   ImportError: attempted relative import with no known parent package

Environment

macOS 10.13.6, Python 3.7.3, pytest 6.0.1

$ pip list                        
Package            Version
------------------ -------
attrs              19.3.0
importlib-metadata 1.7.0
iniconfig          1.0.1
more-itertools     8.4.0
packaging          20.4
pip                20.2.2
pluggy             0.13.1
py                 1.9.0
pyparsing          2.4.7
pytest             6.0.1
setuptools         49.2.0
six                1.15.0
toml               0.10.1
wheel              0.34.2
zipp               3.1.0