Remove the remote repository-access-through-zmq support
Modern methods such as the rqlcontroller cube + the cwclientlib library are the way forward.
Closes #2919297.
What's new in CubicWeb 3.21?
============================
Code movement
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The cubicweb.web.views.timeline module (providing the timeline-json, timeline
and static-timeline views) has moved to a standalone cube_
.. _cube: https://www.cubicweb.org/project/cubicweb-timeline
Deprecated code drops
---------------------
* the user_callback api has been removed; people should use plain
ajax functions instead
* the `Pyro` and `Zmq-pickle` remote repository access methods have
been entirely removed (emerging alternatives such as rqlcontroller
and cwclientlib should be used instead)