fix: correct sync iterator type of asyncio.as_completed to yield Coroutine

[armorbreak001]

Summary

Fixes #15646 — as_completed() sync iteration yields Coroutine, not Future.

Problem

When iterating over asyncio.as_completed() with a regular for loop (not async for), it yields coroutines at runtime. However, the stub typed the sync iterator as Iterator[Future[_T]], which allows calling .result() without a type error — but this crashes at runtime with AttributeError: 'coroutine' object has no attribute 'result'.

Confirmed on CPython 3.12:

>>> import asyncio
>>> it = asyncio.as_completed([asyncio.sleep(0)])
>>> type(iter(it).__next__())
<class 'coroutine'>

Changes

stdlib/asyncio/tasks.pyi:

Version	Before	After
≥3.13	_SyncAndAsyncIterator[Future[_T]] (sync=Iterator[Future])	_SyncAndAsyncIterator[_T] where sync path is Iterator[Coroutine[Any, Any, _T]]
3.10–3.12	Iterator[Future[_T]]	Unchanged (kept for backward compat)
<3.10	Iterator[Future[_T]] (with loop=)	Unchanged

The 3.13+ _SyncAndAsyncIterator Protocol now correctly reflects:

• Sync (__iter__): yields Coroutine[Any, Any, T]
• Async (__aiter__): yields Future[T]

Scope Limitation

The fix is limited to Python 3.13+ only. While the same runtime behavior exists in older versions, changing those stubs breaks existing code (confirmed via mypy_primer) that assigns sync-iter results to Future/Task annotated variables — even though such code would crash at runtime when calling .result(). The typeshed team can decide whether to accept that breakage in a future major version bump.

Verification

• Pyright: 0 errors on modified file
• Runtime confirmed: iter(as_completed(...)).__next__() returns coroutine

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas)
- pandas/core/computation/ops.py:328: error: Need type annotation for "_binary_ops_dict" (hint: "_binary_ops_dict: dict[<type>, <type>] = ...")  [var-annotated]