Still getting ImportPathMismatchError when using tox and fixtures conftest.py

[astrojuanlu]

I am observing a weird situation that makes me question my pytest setup. So far my tests were working well, but today I decided to add some fixtures to conftest.py that actually import the library code, and started getting these errors when running tox:

_________________________________________________________________________________________ ERROR collecting src/poliastro/core/thrust/change_argp.py __________________________________________________________________________________________
.tox/py3-fast/lib/python3.8/site-packages/pytest_doctestplus/plugin.py:246: in collect                                                                                                                                                        
    module = import_path(fspath, root=self.config.rootpath)                                                                                                                                                                                   
.tox/py3-fast/lib/python3.8/site-packages/_pytest/pathlib.py:556: in import_path                                                                                                                                                              
    raise ImportPathMismatchError(module_name, module_file, path)                                                                                                                                                                             
E   _pytest.pathlib.ImportPathMismatchError: ('poliastro.core.thrust.change_argp', '/home/juanlu/Projects/LSF/poliastro/library/.tox/py3-fast/lib/python3.8/site-packages/poliastro/core/thrust/change_argp.py', PosixPath('/home/juanlu/Projects/LSF/poliastro/library/src/poliastro/core/thrust/change_argp.py')) 

There are no __init__.py files under my tests directory, but I acknowledge that maybe it's a weird interaction between tox, pytest, and pytest-doctestplus. The error disappears if I remove my library imports from conftest.py.

This is probably similar to what @matthewfeickert observed in scikit-hep/pyhf#1467 and I solved it in the same way: removing src from testpaths. I'm aware there's also PY_IGNORE_IMPORTMISMATCH=1, but it feels kind of hacky.

Basically, my question is: is this a bug? A feature? Am I doing anything wrong by removing src from testpaths (like suddenly disabling doctests without noticing)? Is PY_IGNORE_IMPORTMISMATCH=1 the only way?