title: What does "error: option --single-version-externally-managed not recognized" indicate?
body: <p>I seem to have suddenly started to encounter the error <code>error: option --single-version-externally-managed not recognized</code> when using <code>pip install</code> with various packages (including <code>PyObjC</code> and <code>astropy</code>).</p>
  <p>I've never seen this error before, but it's now also showing up on travis-ci builds for which nothing has changed.</p>
  <p>Does this error indicate an out of date distribution?</p>
  <p>Some incorrectly specified option in <code>setup.py</code>?</p>
  <p>Something else entirely?</p>