title: Sphinx coverage generates empty python.txt file
body: <p>I'm using <a href="http://sphinx-doc.org/" rel="noreferrer">Sphinx</a> to document my project. I've enabled the <a href="http://sphinx-doc.org/ext/coverage.html" rel="noreferrer">sphinx.ext.coverage</a> extension in my configuration file. I'm running the following command to generate the coverage report (as guided by <a href="https://stackoverflow.com/questions/5179010/sphinx-buildbot-and-code-coverage-how-do-they-integrate">this answer</a>:</p>
  
  <pre><code>bash-3.2$ sphinx-build -v -b coverage . _build/coverage/
  Making output directory...
  Running Sphinx v1.2.3
  loading pickled environment... not yet created
  building [coverage]: coverage overview
  updating environment: 10 added, 0 changed, 0 removed
  ...
  reading sources... [100%] modules
  looking for now-outdated files... none found
  pickling environment... done
  checking consistency... /some/path/modules.rst:: WARNING: document isn't included in any toctree
  done
  build succeeded, 17 warnings.
  </code></pre>
  
  <p>If I look inside _build/coverage/python.txt, all I see is:</p>
  
  <pre><code>Undocumented Python objects
  ===========================
  </code></pre>
  
  <p>This isn't quite right, since there are definitely parts of my project that aren't documented.</p>
  
  <p>Somebody <a href="https://groups.google.com/forum/#!topic/sphinx-users/c_NHZpK28QE" rel="noreferrer">posted</a> about this on the sphinx mailing list, but didn't get any replies.</p>
  
  <p>Does anybody know what the problem is? How do I fix it?</p>