Summary

• Adds asyncio.open_file() — async equivalent of builtin open(), delegates to a thread via asyncio.to_thread()
• Adds asyncio.AsyncFile — async wrapper around synchronous file objects with explicit async I/O methods (read, write, seek, tell, flush, truncate, readline, readlines, writelines, read1, peek, aclose) plus __getattr__ passthrough for sync attributes, async with, and async for line in f support
• Adds asyncio.wrap_file() — wraps an already-open file-like object as AsyncFile
• Adds asyncio.Path — async wrapper around pathlib.Path with async I/O methods and sync property/method passthrough

This follows the proven Trio/anyio pattern of delegating blocking file operations to threads. All async methods use asyncio.to_thread() for thread delegation, reusing existing asyncio infrastructure and propagating contextvars.

Part of the proposal to adopt proven anyio/Trio patterns natively into asyncio (Tier 2, item 2.3). Tracking issue: #145370.

Design decisions

• Explicit async methods for I/O operations (not dynamic __getattr__ dispatch) — clearer for stdlib
• __getattr__ passthrough for sync attributes (name, mode, closed, etc.) — avoids listing dozens of properties
• Wrapper pattern for Path (like anyio) rather than subclassing pathlib.Path — avoids coupling to pathlib internals
• Sentinel-based async iterators for iterdir()/glob()/rglob()/walk() — avoids StopIteration-in-Future issues
• _async_method/_async_method_path helpers to reduce boilerplate in Path (like Trio)

Test plan

• 55 tests covering AsyncFile and Path — all passing
• Verify ./python -m test test_asyncio.test_fileio -v passes