docs: fix CHEATSHEET.rst broken commands (test-requirements.txt, setup.py)

[VedantBhayani]

Fixes broken commands in CHEATSHEET.rst that referenced deleted files:

• test-requirements.txt removed → deps now in pyproject.toml [dependency-groups].test
• setup.py removed → project uses pyproject.toml + setuptools build backend

Updated commands:

• pip install -r test-requirements.txt → pip install -e .[test]
• yapf -rpd setup.py sniffio → yapf -rpd sniffio
• python setup.py sdist bdist_wheel → python -m build

Verified: pip install -e .[test], yapf -rpd sniffio, python -m build, pytest sniffio (3 passed, 1 skipped), pip install pytest pytest-cov curio all work.