Remove unused Python imports and dependencies

[floze-the-genius]

Summary

• remove unused Python imports across tutorial participants and test tooling
• drop matplotlib and numpy from the two participants that no longer use them
• regenerate the affected reference requirements with the repository tool
• preserve the Axes3D side-effect import needed by supported Matplotlib versions before 3.2

Testing

• uvx ruff check . --select F401
• compile every changed Python file with python3 -m py_compile
• run the autopep8 and size pre-commit hooks on the changed files
• run print_metadata.py, print_test_suites.py, and systemtests.py --help
• check both updated reference requirements with --check --fail-on-outdated

The repository-wide check-names hook also ran, but reports pre-existing permalink errors in unchanged README files.

Checklist

• Added changelog-entries/896.md
• Regenerated sibling requirements-reference.txt files for changed requirements

Fixes #833