feat: support pytest 9's new config

[henryiii]

Fix #678.

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[Copilot]

Pull Request Overview

This PR adds support for pytest 9+ configuration formats to the repository review checks. Pytest 9 introduces native TOML configuration (without the ini_options wrapper) in pyproject.toml, as well as standalone pytest.toml and .pytest.toml files.

Key changes:

• Introduces PytestFile enum to distinguish between legacy (6+), modern (9+), and standalone TOML config locations
• Updates all pytest-related checks (PP301-PP309) to work with the new configuration formats
• Adds comprehensive test coverage for the new configuration styles

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/sp_repo_review/checks/pyproject.py	Adds PytestFile enum and pytest() fixture function; updates all PP30x checks to accept pytest fixture tuple
tests/test_pyproject.py	Updates existing tests to use new fixture format; adds tests for modern pytest configs and fixture function
pyproject.toml	Registers new pytest fixture in entry points
docs/pages/guides/pytest.md	Documents both legacy (6+) and modern (9+) pytest configuration formats
README.md	Updates check descriptions for PP302 and PP305

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