title: pytest fails to import module installed via pip and containing underscores
body: <p>I am used to see import errors of my own modules.
  But this time, this is about modules installed via pip and located in site-packages.</p>
  
  <p>Here is a test file expurged from everything but just the imports :</p>
  
  <pre><code>import flask
  import pygments
  from flask_restful import Resource, Api
  #from weather_tool import *
  #from flask_restless import *
  </code></pre>
  
  <p>while running pytest :</p>
  
  <pre><code>Traceback:
  test_MonAPPflaskRest.py:3: in &lt;module&gt;
  from flask_restful import Resource, Api
  </code></pre>
  
  <p>E   ModuleNotFoundError: No module named 'flask_restful'</p>
  
  <p><strong>Actually, every module with an underscore will fail !!!</strong></p>
  
  <p>flask_restless will fail to import to.</p>
  
  <p>But it work when executed outside of pytest, or simply on the python shell...</p>