Use the list of sources instead of an iterator in update-feeds looping task
This prevents RuntimeError due to dictionary size change that may occur (as a
result of a new source being added) during iteration.
Closes #3155843.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html