Treat integer Path.open argument as buffering

[binggao1230]

Fixes #128

Summary

• update Path.open() so an integer second positional argument is treated as buffering instead of encoding
• preserve the existing compatibility path where a non-integer second positional argument is still treated as an encoding
• add a regression test for open("r", 1, encoding="utf-8")

Tests

• Red before the fix: PYTHONPATH=. uv run --python 3.11 --with '.[test]' pytest tests/test_path.py::TestPath::test_open -q failed with _extract_text_encoding() got multiple values for argument 'encoding'\n- PYTHONPATH=. uv run --python 3.11 --with '.[test]' pytest tests/test_path.py::TestPath::test_open -q\n- PYTHONPATH=. uv run --python 3.11 --with '.[test]' pytest tests/test_path.py::TestPath::test_open_encoding_utf16 tests/test_path.py::TestPath::test_open_encoding_errors tests/test_path.py::TestPath::test_open_binary_invalid_args tests/test_path.py::TestPath::test_read -q\n- PYTHONPATH=. uv run --python 3.11 --with '.[test]' pytest -q\n- PYTHONPATH=. uv run --python 3.11 --with '.[test,check]' pytest --ruff -q\n- PYTHONPATH=. uv run --python 3.11 --with mypy mypy zipp\n- git diff --check\n\nI also ran PYTHONPATH=. uv run --python 3.11 --with '.[test,type]' pytest --mypy -q; it currently reports existing test-suite typing issues while pyproject.toml has the mypy pytest enabler disabled.\n\nAI assistance was used under my direction.