Align Path.open positional arguments

[lin-hongkuan]

Summary

• align Path.open() positional parameters with pathlib.Path.open() (buffering, encoding, errors, newline)
• keep the existing legacy Path.open("r", "utf-8") positional encoding form working
• add regression coverage for Path.open("r", 1) and for the public signature order

Fixes #128

Testing

• python -m pytest tests/test_path.py -q
• python -m pytest -q --ignore=tests/test_complexity.py
• python -m ruff check zipp/__init__.py tests/test_path.py
• git diff --check

I also ran python -m pytest -q; it failed only in two existing flaky big-O performance tests in tests/test_complexity.py (test_glob_depth and test_implied_dirs_performance).