[schema] Let RQLConstraint inherit from BaseRQLConstraint instead of RQLVocabularyConstraint There is apparently no reason for RQLConstraint to inherit from RQLVocabularyConstraint as it gets its `repo_check` method from `RepoEnforcedRQLConstraintMixIn` and RQLVocabularyConstraint (which inherits from BaseRQLConstraint) actually only overrides that method. So simplify inheritance by removing a useless level of indirection. Adjust cw_unrelated_rql accordingly, where it was that RQLConstraint inherited from RQLVocabularyConstraint.

from cubicweb.schemas.base import in_group, CWSourceSchemaConfig
# copy __permissions__ to avoid modifying a shared dictionary
in_group.__permissions__ = in_group.__permissions__.copy()
in_group.__permissions__['read'] = ('managers',)

cw_for_source = CWSourceSchemaConfig.get_relation('cw_for_source')
cw_for_source.__permissions__ = {'read': ('managers', 'users'),
                                 'add': ('managers',),
                                 'delete': ('managers',)}