This commit changes the way we obtain the original
Products.ERP5Type.Base.Base.__setstate__ method (before it is
monkey-patched (1)).

Previously, Products.ERP5Type.Base.Base.__setstate__ was inherited from
persistent.Persistent.

Since the previous commit, Products.ERP5Type.Base.Base overrides it with
its own method.

So the test was no longer looking at the right thing.

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(1): See the code inside product/ERP5Type/dynamic/lazy_class.py and
product/ERP5Type/dynamic/persistent_migration.py