title: Pytest fails on Travis: cannot find main package
body: <p>I recently made a large swath of changes to a Python 3 project on Github, largely documentation and docstrings. </p>
  
  <p>The tests pass locally, but when I push the branch to Github, Travis CI fails: </p>
  
  <pre><code>$ pytest
  Traceback (most recent call last):
    File "/home/travis/virtualenv/python3.5.5/lib/python3.5/site-packages/_pytest/config.py", line 342, in _getconftestmodules
      return self._path2confmods[path]
  KeyError: local('/home/travis/build/scossu/lakesuperior')
  During handling of the above exception, another exception occurred:
  Traceback (most recent call last):
    File "/home/travis/virtualenv/python3.5.5/lib/python3.5/site-packages/_pytest/config.py", line 373, in _importconftest
      return self._conftestpath2mod[conftestpath]
  KeyError: local('/home/travis/build/scossu/lakesuperior/conftest.py')
  During handling of the above exception, another exception occurred:
  Traceback (most recent call last):
    File "/home/travis/virtualenv/python3.5.5/lib/python3.5/site-packages/_pytest/config.py", line 379, in _importconftest
      mod = conftestpath.pyimport()
    File "/home/travis/virtualenv/python3.5.5/lib/python3.5/site-packages/py/_path/local.py", line 668, in pyimport
      __import__(modname)
    File "/home/travis/virtualenv/python3.5.5/lib/python3.5/site-packages/_pytest/assertion/rewrite.py", line 212, in load_module
      py.builtin.exec_(co, mod.__dict__)
    File "/home/travis/build/scossu/lakesuperior/conftest.py", line 6, in &lt;module&gt;
      from lakesuperior.config_parser import test_config
  ImportError: No module named 'lakesuperior.config_parser'
  ERROR: could not load /home/travis/build/scossu/lakesuperior/conftest.py
  The command "pytest" exited with 4.
  </code></pre>
  
  <p><code>lakesuperior</code> is the main application package.</p>
  
  <p>My <code>.travis.yml</code>: </p>
  
  <pre><code>language: python
  python:
    - "3.5"
    - "3.6"
  install:
    - pip install -r requirements.txt
    - coilmq&amp;
  script:
  - pytest tests
  </code></pre>
  
  <p>The only thing that I think may have affected the import issue is that I added some <code>__init__.py</code> files to subpackages in order for Sphinx to discover API docs. I am not sure how that may have messed up CI though. </p>
  
  <p>I verified that the previous commit passes all tests as it used to. </p>
  
  <p>Has anybody encountered this scenario before? Any hints?</p>
  
  <p>Thanks.</p>