[schema] store default attribute values in a Bytes field, allowing python objects as default values
Notes:
* Binary objects grow two methods
* we depend on a newer yams version
* the code is quite simplified as a result of the whole change
* a very old and nasty bug where bool(default values) evaluating to
False not being properly migrated is killed
Closes #2414591.
What's new in CubicWeb 3.18?
============================
New functionalities
--------------------
* add a security debugging tool
(see `#2920304 <http://www.cubicweb.org/2920304>`_)
API changes
-----------
* not really an API change, but the entity permission checks are now
systematically deferred to an operation, instead of a) trying in a
hook and b) if it failed, retrying later in an operation
Deprecation
---------------------
Deprecated Code Drops
----------------------
* ``ldapuser`` have been dropped; use ``ldapfeed`` now
(see `#2936496 <http://www.cubicweb.org/2936496>`_)
* action ``GotRhythm`` was removed, make sure you do not
import it in your cubes (even to unregister it)
(see `#3093362 <http://www.cubicweb.org/3093362>`_)
* all 3.8 backward compat is gone
* all 3.9 backward compat (including the javascript side) is gone