Dictionary interface of OSV/Model class conflicts with OSV methods

[Sébastien Alix]

As described here: https://answers.launchpad.net/oerplib/+question/226449

For instance, 'get()' method provided by the "collections.Mapping" (which the 'oerplib.service.osv.Model' class inherits from) conflicts with the 'get()' method of the 'ir.sequence' model:

>>> oerp.get('ir.sequence').get('stock.lot.serial') # Crash

To avoid any problems, it should be convenient to drop "collections.Mapping" inheritance.