Remove external dependency on 'mock'

[padraic-shafer]

This PR removes an unnecessary external dependency on 'mock'. It now uses unittest.mock instead.

[Copilot]

Pull request overview

This PR removes the external mock dependency in favor of the built-in unittest.mock module, which has been available since Python 3.3. The change reduces external dependencies and modernizes the test infrastructure.

• Updated tests/python/requirements-dev.txt to remove the mock package
• Changed the import statement in tests/python/operator_test.py from import mock to from unittest import mock

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/python/requirements-dev.txt	Removed the external mock dependency (note: also includes an unrelated commented dm3_lib line)
tests/python/operator_test.py	Updated import statement to use unittest.mock instead of the external mock package

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