Rm attrs

[Jelleas]

Per @cmlsharp's comment in #368, Python 3.7 brought with it dataclasses that can directly replace attrs. This pr replaces the only uses of attrs in check50 with dataclasses and removes attrs as a dependency.

Opted to use python 3.10 as the 4.0.0-dev branch is already using features from Python 3.10. Such as match, and importlib.resources.files that was added in Python 3.9.

Used python 3.10 syntax for type hints (notably the use of |, and list instead of typing.List).
Set slots=True which is only supported from 3.10 onwards.

[Copilot]

Pull request overview

This PR modernizes the codebase by replacing the attrs library with Python's built-in dataclasses module, which has been available since Python 3.7 and provides equivalent functionality.

• Removes attrs as a dependency
• Updates minimum Python version from 3.8 to 3.10 to leverage modern type hint syntax
• Converts the CheckResult class from attrs to dataclasses with proper type annotations

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
setup.py	Removes attrs>=18 from dependencies and updates minimum Python requirement to 3.10
check50/runner.py	Converts CheckResult from attrs to dataclass with Python 3.10 type hints, updates attr.fields() to dataclasses.fields()
check50/main.py	Replaces attr.asdict() with dataclasses.asdict() for serialization

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[rongxin-liu]

Thank you! @Jelleas