fix interface_selector bug
If a view using this selector defines an `accepts` attribute, the
view should be selectable only if the entity is of one of the
accepted types (+need to consider schema inheritance)
class ToTo(EntityType):
permissions = {
'read': ('managers',),
'add': ('managers',),
'update': ('managers',),
'delete': ('managers',),
}
attr = String()
class attr(RelationType):
permissions = {
'read': ('managers', ),
'add': ('managers', RRQLExpression('S bla Y'),),
'delete': ('managers',),
}