Remove imports from within test functions

[bielsnohr]

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PEP8 specifies that imports should be put at the top of a Python file: https://peps.python.org/pep-0008/#imports

Therefore, linters will complain for the tests explained in Episode 2, Section 2, "Scaling Up Unit Testing".

We should group all imports at the top of the test files unless we want this to be a teachable linting moment.