Python/tox Install a dependency as editable

In tox.ini, you specify the packages that you want tox to install in the virtualenvs it creates.

[testenv]
deps =
    mock
    pytest
commands =
    python setup.py test -q
    python setup.py flake8

This example tells tox to install mock and pytest in to each virtualenv before running the tests. Tox will use pip to install those dependencies from PyPI.

How do I tell tox to pip install -e one dependency from a local checkout instead of from PyPI? I still want the rest of the dependencies to be installed from PyPI.