Use asyncio.timeout on Python 3.11+

[8rief]

Summary

• use asyncio.timeout for ApplicationCommunicator on Python 3.11+
• keep the existing vendored timeout unchanged as the Python 3.10 fallback
• cover cancellation-count restoration after a handled receive_output timeout

Related to #504.

Verification

The regression test failed before the production change because the handled timeout left the current task's cancellation count at 1 instead of restoring it to 0.

• pytest tests/test_testing.py -q
• pytest -q — 98 passed, 1 xfailed on Python 3.12
• mypy .
• pre-commit run --files asgiref/testing.py tests/test_testing.py
• Python 3.10 container — 95 passed, 2 skipped, 1 xfailed; mypy passed
• Python 3.11 and 3.14 containers — focused tests passed
• Python 3.14 container — 98 passed, 1 xfailed; mypy passed

[carltongibson]

@graingert WDYT?

[carltongibson]

We're dropping Python 3.10 very shortly. I wonder if the correct approach here isn't to switch testing.py to asyncio.timeout instead, and deprecate timeout.py here. (We might merge this anyway...)

[carltongibson]

Needs a rebase.

[graingert]

I'd be very tempted to use the built in asyncio.timeout where available and leave this timeout alone for 3.10

[8rief]

Rebased and revised this along the suggested compatibility boundary. ApplicationCommunicator now uses asyncio.timeout on Python 3.11+, while Python 3.10 keeps the existing vendored fallback unchanged.

I moved the regression coverage to ApplicationCommunicator.receive_output; it confirms that a handled timeout no longer leaves the caller task in a cancelling state. The full test suite and mypy pass on Python 3.10 and 3.14, with focused checks on Python 3.11 as well.