Since pypi is behind a CDN mirrors became deprecated.
http://www.python.org/dev/peps/pep-0449/#plan-for-deprecation-removal

Yesterday, when travis-ci tried to build my this PR, one of them hanged, asking pip to login...
https://travis-ci.org/ticosax/urllib3/jobs/18197138

So I thought it was for the best.
I can remove the commit if you like.