utils/backtrace: enable strict typing

[alebcay]

• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same change?
• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes? Here's an example.
• [x] Have you successfully run brew style with your changes locally?
• [x] Have you successfully run brew typecheck with your changes locally?
• [x] Have you successfully run brew tests with your changes locally?

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[MikeMcQuaid]

Thanks @alebcay!