gh-150060: add asyncio.Task.cancelling_since() to expose first cancellation timestamp

[prince8273]

Summary

Add Task.cancelling_since() — a new method on asyncio.Task that returns
the event loop time (float) when cancel() was first called, or None if
cancellation has not been requested.

Motivation

Task.cancelling() exposes how many cancellation requests were made, but not
when the first request happened. This makes it impossible to measure
cancellation latency, debug timeout handling, or emit metrics in long-running
async systems.

Changes

• Modules/_asynciomodule.c — new struct field, init, clear, traverse, first-cancel timestamp capture, Argument Clinic definition, method impl, method table entry
• Modules/clinic/_asynciomodule.c.h — Clinic-generated wrapper
• Lib/asyncio/tasks.py — pure-Python _PyTask mirror
• Lib/test/test_asyncio/test_tasks.py — 3 regression tests
• Doc/library/asyncio-task.rst — API docs with .. versionadded:: 3.16
• Misc/NEWS.d/ — blurb entry for asyncio: add Task.cancelling_since() to expose first cancellation timestamp #150060

Smoke test

before: None
after: 68086.5663514
stable: True

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #32756056 | 📁 Comparing 9724051 against main (ba0aca3)

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2 files changed

± library/asyncio-task.html
± whatsnew/changelog.html