no-data-collected when running tests with tox but not when running tests with pytest

I'm using tox to run unit tests with pytest. When I run the tests with tox, it runs the tests and shows the tree of my src directory but prints CoverageWarning: No data was collected. (no-data-collected). When I run with pytest I get the exact same output except without the error and the coverage numbers get printed as expected. My tox.ini looks like this:

[tox]
envlist = py39

[testenv]
deps = 
    pytest
    pytest-cov
    -rrequirements.txt
    -rrequirements.test

commands = pytest {posargs}

[pytest]
addopts =
    -v
    --cov=src
    tests/

I've tried other solutions mentioned on SO, such as

• Making sure each directory in src has a __init__.py
• Making sure the test directory has a __init__.py
• Removing execute permissions for every python file in src
• Adding include / omit statements to tox.ini
• Removing modules in src with "test" in the name

I changed my tox.ini to

[tox]
envlist = py39

[testenv]
deps = 
    pytest
    pytest-cov
    -rrequirements.txt
    -rrequirements.test

commands = pytest --cov=src {posargs:tests}

[pytest]
pythonpath = ./src

which solved the issue. I guess pytest needed the path to be explicitly passed in.