How to run tox command with posargs?

I have a tox.ini that uses the py.test runner.  When I configure a run configuration to run only a specific test with `-- path/to/my/test_file`  PyCharm never passes the extra arguments in {posargs} and all the tests run.  

Here is my tox.ini:

[tox]
envlist = py27
skipsdist = True

[testenv]
deps =
  -r{toxinidir}/requirements.txt
  -r{toxinidir}/requirements-test.txt
commands =
    py.test {posargs}