Interesting. As of last night, I'm no longer seeing 'gzip' encoding and the test passes for me.  But I see some of the buildbots intermittently failing.  Looking at the headers for www.python.org/robots.txt, it appears that the file is being served from a varnish cache and from a CDN so there may be different responses depending on which server responds.  

>>> r1.getheaders()
[('Server', 'nginx'), ('Content-Type', 'text/plain'), ('X-Frame-Options', 'SAMEORIGIN'), ('Content-Length', '690'), ('Accept-Ranges', 'bytes'), ('Date', 'Fri, 21 Feb 2014 23:53:23 GMT'), ('Via', '1.1 varnish'), ('Age', '2858'), ('Connection', 'keep-alive'), ('X-Served-By', 'cache-sv62-SJC3'), ('X-Cache', 'HIT'), ('X-Cache-Hits', '1')]

In any case, supporting gzip would be a good idea but tests will need to have a more repeatable URL.