title: KeyError in module 'threading' after a successful py.test run
body: <p>I'm running a set of tests with py.test. They pass. Yippie! But I'm getting this message:</p>
  
  <pre><code>Exception KeyError: KeyError(4427427920,) in &lt;module 'threading' from '/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.pyc'&gt; ignored
  </code></pre>
  
  <p>How should I go about tracking down the source of that? (I'm not using threading directly, but am using gevent.)</p>