Fixes broken unit test for materializers

[skrawcz]

The name it was getting before was ABCmeta... rather than the fully qualified class name. It seemed the simplest to just str() the object as it includes the fully qualified name in the class that way.

Changes

• fixes broken unit test for materializer registration

How I tested this

• locally

Notes

• not sure how this worked in the first place...?

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.