* New: Isobar::PyAttributesHolder, a PyObject with only a dictionary
to hold the attributes associated to a DBo.
* New: Isobar::PyHolderProperty, the Property that encapsulate
PyAttributesholder.
* New: Isobar::PythonAttributes, the extension to simplify the
management of the PyAttributesholder.
* Change: In PyEntity, now use a dedicated tp_getattro and tp_setattro
to delegate the Python attribute access towars the PyAttributesholder.
* New: In hurricane/doc & documentation, update docs regarding Python
attributes managment.
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