flake8 CI failure

[j-c-c]

Seeing some new linting failures: https://github.com/ComputationalCryoEM/ASPIRE-Python/actions/runs/34522684306/job/103023787567?pr=1391

Will put in a patch shortly.

[j-c-c]

Looks like the flake8 plugin flake8-bugbear had a release 2 days ago that is now catching an isinstance call that is not being used. We are just missing an assert to check the isinstance.