Replace flake8 with ruff for linting and formatting

[Dianababaei]

Summary

Migrate from flake8 to ruff for faster linting and code formatting. Ruff provides equivalent rule coverage while offering better performance and a unified formatting solution.

Changes

• Replace flake8 with ruff in .pre-commit-config.yaml (v0.8.4)
• Add ruff configuration to pyproject.toml with equivalent rules (E, W, F)
• Migrate per-file ignores from setup.cfg to pyproject.toml
• Add ruff-format hook for automatic code formatting
• Add ruff>=0.1.0 to development dependencies
• Remove flake8 configuration from setup.cfg

Testing

• Verify pre-commit hooks run without errors
• Confirm ruff linting and formatting produce expected output
• Check that per-file ignores are respected (especially src/requests/__init__.py and compat.py files)

Notes

Ruff configuration targets Python 3.9+ and maintains feature parity with previous flake8 setup. The --fix argument enables automatic fixing of violations where possible.