Added pytest assertions

[GmoZen]

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Partially addresses https://github.com/AcademySoftwareFoundation/OpenTimelineIO/issues/1228

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Summarize your change.

Replaced unittest assertions with Pytest assertions (except those for multi-line strings, as unittests assertions were more descriptive). We're migrating unit tests from vanilla unittest to Pytest to better be inline with the greater Python community.

Add a list of changes, and note any that might need special attention during review.

Reference associated tests.

test_clip.py

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• :white_check_mark: login: GmoZen / name: Guillermo Zendejas (da5549b11e6a13389a3e6a86f1a4d243197c481d)

[jminor]

I like the improved readability of the new test cases.

The CI is complaining about linting errors. You can run the linter locally to ensure that your change will pass that check, via make lint. After resolving those, you can push your changes to the same branch you've been using and this PR will update & re-run the CI checks.

Also, let us know if you need help with DCO or EasyCLA. We know those can be confusing for new contributors & we're happy to explain what those are all about.

[jminor]

I think the remaining linter issues are not your fault. See this: https://github.com/AcademySoftwareFoundation/OpenTimelineIO/issues/1369