Add asyncio.CancelScope with level-triggered cancellation

[kovan]

Summary

• Add asyncio.CancelScope, an async context manager providing level-triggered cancellation for asyncio
• Once cancelled (via cancel() or deadline expiry), every subsequent await inside the scope raises CancelledError until the scope exits — the coroutine cannot simply catch-and-ignore
• Implements both Python (_PyTask) and C (_CTask) Task changes
• Existing edge-triggered mechanisms (cancel()/uncancel(), Timeout, TaskGroup) remain completely unchanged; code not using CancelScope has zero overhead

Motivated by the discuss.python.org proposal to adopt proven Trio/anyio patterns into asyncio. Tracking issue: #145370.

New public API

• asyncio.CancelScope(*, deadline=None, shield=False) — async context manager
• asyncio.cancel_scope(delay, *, shield=False) — convenience with relative delay
• asyncio.cancel_scope_at(when, *, shield=False) — convenience with absolute time

Properties: deadline (r/w), shield (r/w), cancel_called (r), cancelled_caught (r)
Methods: cancel(), reschedule(deadline)

How it works

CancelScope pushes/pops itself on a per-task _current_cancel_scope linked-list stack. In Task.__step, after the existing _must_cancel check, a new elif checks whether the current scope is cancelled and not shielded — if so, it re-injects CancelledError. The fast path (_current_cancel_scope is None) is a single pointer check with zero overhead.

Test plan

• 51 new tests in test_asyncio.test_cancelscope covering both _PyTask and _CTask
• Level-triggered re-injection (5 successive catches in a loop)
• Deadline/timeout, cancel_scope()/cancel_scope_at() convenience APIs
• shield=True blocking re-injection
• Nested scopes (inner cancelled, outer cancelled, both cancelled)
• cancelled_caught semantics
• TaskGroup interaction
• Edge-triggered task.cancel() unchanged
• Full test_asyncio suite passes (2,646 tests, 0 failures)

[ZeroIntensity]

See my comment.