Tox+Poetry Pytest error related to imports

[incoggnito]

Puh, i really struggle getting poetry and tox working together.

When i run my pytest directly:

poetry run pytest --cov=src tests/

Everything works fine.
Running tox fails on importing the packages correctly.

poetry run tox -e py38

Here is my tox config:

[flake8]
application-import-names =
    atoolbox
    tests
docstring-convention = google
ignore =
    D202
    E203
    W503
import-order-style = google
max-complexity = 10
max-line-length = 80
per-file-ignores =
    scripts/*:B008

[testenv]
commands =
    poetry install -vv
    poetry run pytest --cov=src tests/
deps = poetry
description = Test Python code.

[testenv:lint]
commands =
    poetry install -vv
    poetry run bandit -ilr src
    poetry run black --check .
    poetry run python scripts/docs.py build
    poetry run flakehell lint scripts src tests
    poetry run mypy scripts src tests
deps = poetry
description = Run static analyzers on Python code.

[tox]
envlist = lint, py37, py38, py39
isolated_build = true

[tox:.package]
basepython = python3

I already reset the pyproject.toml as recommended in the docs.

[build-system]
build-backend = "poetry.masonry.api"
#requires = ["poetry>=1.0.0"]
requires = ["poetry >= 0.12, <1"]

Maybe there is some error related to my package structure:

| > src
| >>atoolbox
| >>>__init__.py
from .filehandler.main import FileHandler
| >>>filehandler
| >>>>__init__.py
from .main import FileHandler
|>>>>main.py
Contains a class called FileHandler
|>tests
|>>filehandler
|>>>test_main.py
|>__init__.py

Here the error:

..\..\..\AppData\Local\Programs\Python\Python38\lib\importlib\__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests\filehandler\test_main.py:13: in <module>
    from atoolbox import FileHandler
.tox\py38\lib\site-packages\atoolbox\__init__.py:3: in <module>
    from .filehandler.main import FileHandler
E   ModuleNotFoundError: No module named 'atoolbox.filehandler'

Any ideas?