Update doctests for power_sum.py

[hollowcrust]

Describe your change:

Contributes to #9718.

I changed the test solve(800, 2) where needed_sum is decreased to 500, as this test has a large search space. The test solve(1000, 10) is not a problem because 2 ** 10 = 1024 and the function terminates quite quickly.

But anyways I feel this test solve(800, 2) is redundant since the last test is solve(400, 2), which is quite similar.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
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• All functions have doctests that pass the automated testing.
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[hollowcrust]

I also moved the import command because originally the code did not pass ruff check.