title: Pytest says 'ModuleNotFoundError' when using tox
body: <p>I have the following project structure:</p>
  
  <pre><code>root
  |- module
    |- module.py
    |- __init__.py
  |- tests
     |- unit
        |- some_test.py
     |- integration
        |- another_test.py
  |- conftest.py
  |- setup.py
  |- tox.ini
  </code></pre>
  
  <p>When I run <code>python3 module/module.py ...</code> it runs as expected. </p>
  
  <p>However, when I execute <code>tox</code>, I get <code>ModuleNotFoundError: No module named 'dateutil'</code>.</p>
  
  <p>In my setup.py I have <code>install_requires=['python-dateutil']</code> and tox.ini has the following (simplified) contents:</p>
  
  <pre><code>[tox]
  envlist   = py{36, 37}
  skipsdist = True
  
  [testenv]
  deps = pytest
  commands = pytest
  </code></pre>
  
  <p>Does anyone have any insight as to why running <code>tox</code> gives me that the module 'dateutil' cannot be found and how to fix it?</p>