o Avoids deprecation warnings in Python 2.6.  See:

  http://docs.python.org/library/subprocess.html#replacing-os-popen-os-popen2-os-popen3


git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@97510 62d5b8a3-27da-0310-9561-8e5933582275