Field instance copying failes before contribute_to_class

[paluh]

Python copy.copy() called on Field instance which is not contributed to class (before contribute_to_class call) raises:

AttributeError: 'SomeField' object has no attribute 'model'

This issue is related to #12977 changeset. It has added db.models.Field.__setstate__ method which expects that Field instance has 'model' attribute (but it is initialised in contribute_to_class method). I think that copying of instance should be safe all the time and this problem can be easily fixed by 'hasattr' check. Should I provide patch with test?

[Russell Keith-Magee]

Duplicate of #13366