[CT-621] Turn on mypy strict flag

[nathaniel-may]

#5171 upgraded mypy to the latest version and changed it to run it on the entire codebase. This ticket is to turn on the --strict flag which dramatically increases the enforced level of code quality.

This will take a large effort to convert the existing code to comply with the strict flag, and should be done iteratively where possible. One strategy would be to run mypy in CI twice: once on as we currently do, and once with the strict flag on a small subset of the codebase that can be expanded with subsequent PRs. Following Imports may make this strategy limiting.

[github-actions]

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