Fix test_serialization

Description

It was required to create a separate file. I tried with just adding if typing.TYPE_CHECKING, but then main plugins file started causing the same problems.

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• reviewer 1

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• Changelog entry and version bump inmanta module release --dev [--major|--minor|--patch] [--changelog-message "<your_changelog_message>"]
• Code is clear and sufficiently documented (classes, methods and function docs, document important assumptions, link to reference materials)
• Sufficient test cases (reproduces the bug/tests the requested feature)
• Correct, in line with design
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