[test] replace some assert{True,False} with more specific method

[test] silence warnings for unittest_entity by upgrading to new 3.19 API

Fix constraint sync during migration - restore constraints lost during merge in test schema. - use constraint_by_eid in BeforeDeleteCWConstraintHook as done in 3.17.14 for BeforeDeleteConstrainedByHook. Fixes handling of multiple constraints of the same type. - make sync_schema_props_perms() delete the CWConstraint entity instead of the constrained_by relation. In 3.19, the latter doesn't automatically result in the former just because the relation is composite. Simplify the constraint migration to delete all removed constraints and recreate new ones even if they share the same type; that optimization made the code more complicated for (AFAICT) no significant reason.

merge 3.18.4 into default server/test/unittest_migractions fails due to interaction between new constraint test (from 3.18) and composite deletion change (in 3.19).

Added tag cubicweb-version-3.18.4, cubicweb-debian-version-3.18.4-1, cubicweb-centos-version-3.18.4-1 for changeset 0176da9bc752

[test/data] Add missing 'add' permission on attribute Silences a warning introduced in 3.18.

[hooks/notification] Fix indentation Follow-up for changeset 1b8552265f3b "[hooks/notification] use a cnx not a session"

[devtools] deprecate CWTC.user()

minor cleanups

some copyright updates

[devtools] properly close open access on tearDown avoiding further warning about living sessions while the repo is turned off. The doc string says: The RepoAccess need to be closed to destroy the associated Session. TestCase usually take care of this aspect for the user. but that wasn't true until now.

[devtools] make comment (a bit) more readable