Add scripts/find_and_fix_pep798.py

[cclauss]

Describe your change

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Adds scripts/find_and_fix_pep798.py, a uv run --script tool that finds (and with --fix rewrites) PEP 798 opportunities such as [y for x in xs for y in x] -> [*x for x in xs] and {k: v for d in ds for k, v in d.items()} -> {**d for d in ds}. Covers list, set, generator and dict comprehensions. Pytests are in tests/test_find_and_fix_pep798.py.

Validation: uvx --with pytest pytest tests/test_find_and_fix_pep798.py scripts/find_and_fix_pep798.py --doctest-modules -> 41 passed; uvx ruff check and uvx ruff format -> clean.

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.
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• All functions have doctests that pass the automated testing.
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• If this pull request resolves one or more open issues, then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".