title: Pytest does not rewrite AssertionError
body: <p>I'm testing a Django project using pytest with the pytest-django and pytest-cov plugins and when I'm telling it to look for tests in a directory (<code>pytest project -vv --cov --ds=proj.tests</code>), it does not rewrite the AssertionError when it happens :</p>
  
  <pre><code>    def test_pytest():
  &gt;      assert 2 == 3
  E      AssertionError
  </code></pre>
  
  <p>However, it still always work in another directory / django app, where it is a single tests.py file, as opposed to a directory containing python files in the above case</p>
  
  <pre><code>&gt;       assert rec.action == 'updater'
  E       assert 'update' == 'updater'
  E         - update
  E         + updater
  E         ?       +
  </code></pre>
  
  <p>Also, I don't know if it helps or just confuse everything, but when looking directly for the file (<code>pytest project/core/tests/pytest.py -vv --cov --ds=proj.tests</code>), it works as expected:</p>
  
  <pre><code>    def test_pytest():
  &gt;       assert 2 == 3
  E       assert 2 == 3
  </code></pre>
  
  <p>It's even stranger as it worked before, and I did not touch any of its configuration (everything is the same for my colleague, allegedly he's on a way older commit).</p>
  
  <p>I'm using tox to automate the tests.<br>
  Here's the pytest section</p>
  
  <pre><code>[pytest]
  python_files =
    tests/*.py
    tests*.py
    test_*.py
  minversion = 3
  </code></pre>
  
  <p>Why does it not work anymore like it should ? Is there anyway to solve this problem ? I don't want to go back to the days of using print :(</p>
  
  <p>I'm using Python 3.5.2, tox 2.4.1, pytest-django 3.0.0 (and its pytest dep) and pytest-cov 2.4.0</p>