title: PIP randomly fails 'Could not find a version that satisfies the requirement' with the same requirements.txt
body: <p>As part of our CI testing we install a virtualenv with some pip packages from a constant requirements.txt file.</p>
  
  <p>this installation process randomly fails from time to time with no apparent reason as the requirements.txt file doesn't change. And each time it's for a different random package.</p>
  
  <p>The CI is on an AWS machine so I don't think it can be an internet issue</p>
  
  <p>The failure looks similar to that (with different package failing):</p>
  
  <pre><code>Collecting django-rest-auth==0.9.3 (from -r requirements.txt (line 7))
  Could not find a version that satisfies the requirement django-rest-auth==0.9.3 (from -r requirements.txt (line 7)) (from versions: )
  No matching distribution found for django-rest-auth==0.9.3 (from -r requirements.txt (line 7))
  </code></pre>
  
  <p>Or</p>
  
  <pre><code>Collecting py&gt;=1.5.0 (from pytest-&gt;-r requirements.txt (line 15))
  Could not find a version that satisfies the requirement py&gt;=1.5.0 (from pytest-&gt;-r requirements.txt (line 15)) (from versions: )
  No matching distribution found for py&gt;=1.5.0 (from pytest-&gt;-r requirements.txt (line 15))
  </code></pre>
  
  <p>EDIT: Tried adding <code>--timeout 30 --retries 15</code> which didn't seem to change anything</p>