title: Make pip install specific version of dependency package over installing latest
body: <p>I have a requirements.txt with packages <code>a</code> and <code>b</code>.</p>
  <ul>
  <li><code>a</code> has a dependency called <code>c</code> with the constraint <code>c==2.3.0</code></li>
  <li><code>b</code> has the same dependency but has the constraint <code>c&gt;=1.2</code></li>
  </ul>
  <p>If <code>c</code>'s latest version available is <code>4.0</code>, pip is installing that. But, I need pip to install <code>2.3.0</code> How can I make pip do that?</p>