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This has been banned since the introduction of Portal Type class. When creating
a new ERP5Form via addERP5Form/ZMI, its MRO:
  * Before:
    <class 'Products.ERP5Form.Form.ERP5Form'>
    <class 'Products.ERP5Type.Base.Base'>
    ...
    <type 'ExtensionClass.Base'>
  * Now:
    <class 'erp5.portal_type.ERP5 Form'>,
    <class 'Products.ERP5Form.Form.ERP5Form'>,
    <class 'Products.ERP5Type.Base.Base'>,
    ...
    <class 'erp5.accessor_holder.property_sheet.SimpleItem'>
    <class 'erp5.accessor_holder.property_sheet.Folder'>
    <class 'erp5.accessor_holder.property_sheet.Base'>
    <class 'erp5.accessor_holder.property_sheet.CategoryCore'>
    <class 'erp5.accessor_holder.BaseAccessorHolder'>
    <class 'Products.ERP5Type.dynamic.portal_type_class.GetAcquireLocalRolesMixIn'>
    <type 'ExtensionClass.Base'>

Thus it was missing many accessors and was working only by chance (or at least
unless these accessors were...