ModuleNotFoundError when tests are in subdirectory of the main package

[Ark-kun]

I git a third-party package with the following structure:

./__init__.py
./conftest.py
./some_code.py
./tests/
./tests/__init__.py
./tests/conftest.py
./tests/test_version.py
./tests/utils/
./tests/utils/__init__.py
./tests/utils/fixtures.py

./tests/conftest.py:

from tests.utils.fixtures import foo

When the CI system runs pytest I get an error:
pytest ./tests/test_version.py

ImportError while loading conftest 'xxx/tests/conftest.py'.
tests/conftest.py:13: in <module>
    from tests.utils.fixtures import foo
E   ModuleNotFoundError: No module named 'tests'

For internal reasons I cannot set PYTHONPATH.

Is there any pytest arg that would help me make this conftest import work?