title: Wagtail dependency issue
body: <p>Using Wagtail 1.13.1 we're suddenly getting an error when building locally using Docker, and the build fails.</p>
  
  <pre><code>Cannot uninstall 'urllib3' It is a distutils installed project and 
  thus we cannot accurately determine which files belong to it which 
  would lead to only a partial uninstall. 
  </code></pre>
  
  <p>Is urllib3 a dependency of Wagtail via Requests?</p>
  
  <p>has anyone else experienced this?</p>
  
  <p>Thanks</p>
  
  <p>Joss</p>