cerbero: Add a dotted progress bar for urllib downloads

Also add a 20s timeout (same as wget) so that we timeout on download
instead of timing out the entire job on CI. The global socket timeout
with CPython is apparently -1, aka never timeout.

Part-of: <gstreamer/cerbero!726>
14 jobs for !726 with backport-mr-724-into-1.18-urllib2-progress in 91 minutes and 3 seconds (queued for 9 seconds)
merge request