[datafeed parser] fix retrieve_url to always return urllib2.urlopen compatible output
as one should expect to be able to call geturl/getcode/info on the returned
object whatever the url he gave.
Related to #3682069
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/