title: py.test does not correctly import modules from conftest - fails with ConftestImportFailure: ModuleNotFound
body: <p><code>pytest</code> fails to import modules even though these modules import fine under the vanilla <code>python</code> interpreter. Especially modules imported from <code>conftest</code> fails to resolve. ImportErrori is raised when pytest tries to rewrite assert information in modules.</p>
  
  <p>An example traceback:</p>
  
  <pre><code>================================================================================================================================================ test session starts ================================================================================================================================================
  platform darwin -- Python 3.6.2, pytest-3.8.1, py-1.5.4, pluggy-0.7.1 -- /yyy/venv/bin/python3.6
  cachedir: .pytest_cache
  rootdir: /yyy/corporategovernance, inifile: setup.cfg
  plugins: timeout-1.3.1, cov-2.5.1, populus-1.9.0, flaky-3.4.0, celery-4.1.0, splinter-1.8.5+headless
  collecting 0 items / 1 errors                                                                                                                                                                                                                                                                                       
  ====================================================================================================================================================== ERRORS =======================================================================================================================================================
  _________________________________________________________________________________________________________________________________ ERROR collecting corporategovernance/__init__.py __________________________________________________________________________________________________________________________________
  ../venv/lib/python3.6/site-packages/_pytest/config/__init__.py:381: in _getconftestmodules
      return self._path2confmods[path]
  E   KeyError: local('/xxx/tests')
  
  During handling of the above exception, another exception occurred:
  ../venv/lib/python3.6/site-packages/_pytest/config/__init__.py:412: in _importconftest
      return self._conftestpath2mod[conftestpath]
  E   KeyError: local('/xxx/tests/conftest.py')
  
  During handling of the above exception, another exception occurred:
  ../venv/lib/python3.6/site-packages/_pytest/config/__init__.py:418: in _importconftest
      mod = conftestpath.pyimport()
  ../venv/lib/python3.6/site-packages/py/_path/local.py:668: in pyimport
      __import__(modname)
  ../venv/lib/python3.6/site-packages/_pytest/assertion/rewrite.py:290: in load_module
      six.exec_(co, mod.__dict__)
  corporategovernance/tests/conftest.py:4: in &lt;module&gt;
      import corporategovernance.backend
  E   ModuleNotFoundError: No module named 'corporategovernance.backend'
  
  During handling of the above exception, another exception occurred:
  ../venv/lib/python3.6/site-packages/_pytest/runner.py:201: in __init__
      self.result = func()
  ../venv/lib/python3.6/site-packages/_pytest/runner.py:261: in &lt;lambda&gt;
      call = CallInfo(lambda: list(collector.collect()), "collect")
  ../venv/lib/python3.6/site-packages/_pytest/python.py:624: in collect
      for x in self._collectfile(path):
  ../venv/lib/python3.6/site-packages/_pytest/python.py:579: in _collectfile
      ihook = self.gethookproxy(path)
  ../venv/lib/python3.6/site-packages/_pytest/python.py:568: in gethookproxy
      my_conftestmodules = pm._getconftestmodules(fspath)
  ../venv/lib/python3.6/site-packages/_pytest/config/__init__.py:395: in _getconftestmodules
      mod = self._importconftest(conftestpath)
  ../venv/lib/python3.6/site-packages/_pytest/config/__init__.py:431: in _importconftest
      raise ConftestImportFailure(conftestpath, sys.exc_info())
  E   _pytest.config.ConftestImportFailure: ModuleNotFoundError("No module named 'corporategovernance.backend'",)
  E     File "/yyy/venv/lib/python3.6/site-packages/_pytest/assertion/rewrite.py", line 290, in load_module
  E       six.exec_(co, mod.__dict__)
  E     File "/xxx/tests/conftest.py", line 4, in &lt;module&gt;
  E       import corporategovernance.backend
  ===Flaky Test Report===
  </code></pre>
  
  <p>How to resolve?</p>