title: How can tox install the modules via the requirements file?
body: <p>Our python project has a requirements.txt file which lists some dependent module. We used to use </p>
  
  <pre><code>pip install -r requirements.txt
  </code></pre>
  
  <p>to install these dependencies. We are now using tox to build up the test environment. My question is that how can we install the modules via requirements.txt directly.</p>
  
  <p>Followings are our tox.ini and requirements.txt:</p>
  
  <p>tox.ini:</p>
  
  <pre><code>[tox]
  envlist=py27
  [testenv]
  deps=pytest
       boto
  commands=py.test
  </code></pre>
  
  <p>rquirements.txt:</p>
  
  <pre><code>boto
  </code></pre>
  
  <p>Is any way to remove the "boto" from tox.ini and add something like</p>
  
  <pre><code>deps_files=requirements.txt
  </code></pre>