title: How do I force pip to use existing versions of dependencies
body: <p>I'm in a <code>virtualenv</code> with the line <code>pytz==2012h</code> in the <code>requirements.txt</code> used by <code>pip</code>. When I run <code>pip install protobuf</code> it fails and the traceback ends with:</p>
  
  <pre><code>pkg_resources.VersionConflict: (pytz 2012h (/home/alex/work/app/venv/lib/python2.7/site-packages), Requirement.parse('pytz&gt;=2010'))
  </code></pre>
  
  <p>which seems silly, surely <code>2012h&gt;=2010</code>? I dived in with <code>pdb</code> and eventually reached this:</p>
  
  <pre><code>268             if not isinstance(prospective, Version):
  269  -&gt;             return False
  </code></pre>
  
  <p>where <code>prospective</code> was <code>&lt;LegacyVersion('2012h')&gt;</code>. So the version comparison fails (i.e. it decides that the requirement isn't satisfied) because it interprets 2012h as a <code>LegacyVersion</code>. Can I get around this? Installing <code>protobuf</code> with <code>--no-deps</code> doesn't work.</p>