Problem: Pointer file creation fails in Python 3

[replaceafill]

Expected behaviour

Pointer files can be created in Python 2 and 3.

Current behaviour

Some of the logic to create pointer files (replicas and encryption) relies on PREMIS agents and events being unique and filtered using sets. See:

• https://github.com/artefactual/archivematica-storage-service/blob/83d7b5a7da79c158cb99bd0e3426b92fdde0d3f0/storage_service/locations/models/package.py#L1169-L1171
• https://github.com/artefactual/archivematica-storage-service/blob/83d7b5a7da79c158cb99bd0e3426b92fdde0d3f0/storage_service/locations/models/package.py#L1253-L1254

However these objects (subclassed from PREMISElement) are not hashable, which is a requirement of set operations, and they implement the __eq__ method which makes them fail in Python 3 with a TypeError: unhashable type.

Steps to reproduce

I found this through the aip-encrypt-mirror AMAUAT tag using the Python 3 branch of the SS.

Your environment (version of Archivematica, OS version, etc)

https://github.com/artefactual-labs/am/commit/67dbcc0fbbf541ed2594c83078a4299c808fe84c using the Python 3 branch of the SS.

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[sromkey]

Tested with RC2, pointer files are being created.