[predicates] keep making `yes` predicate importable from cubicweb (closes #2790319) The `yes` predicate was moved out in CubicWeb 3.15 but it should have remained importable from the cubicweb.predicates module, for bw compat reasons and also because logilab.common.registry is an implementation detail.

option_group_changed('cleanup-session-time', 'web', 'main')