Products.ERP5Type.mixin.temporary.TemporaryDocumentMixin and
Products.ERP5Type.Base.TempBase implementations were a bit different:

 - getTitle was acquiring title in TemporaryDocumentMixin, although this
 was probably never acquiring in practice because most documents have
 a title.
 - isIndexable was an int and not a PropertyConstantGetter like it is
 supposed to be.

Change TemporaryDocumentMixin to behave like TempBase here.

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