bugfix: fix cancelled tasks pending forever

[ngtrunghuan]

When an AsyncHandle is cancelled before the _main_loop is able to dequeue it (often due to heavy CPU load), trio-asyncio currently leaves the underlying asyncio.Future hanging in the PENDING state forever, preventing the cancellation to travel upwards back into the trio loop, leading to us missing task cancellation/completion. Specifically, _main_loop_one greedily drops it at if obj._cancelled: return without running _run, leaving the result_future pending forever. The fix is to always run AsyncHandle._run.

The change includes

• a new failing test in tests/test_trio_asyncio.py::test_async_handle_cancelled_before_dequeue that forces a cancellation before the _main_loop has a chance to dequeue the pending task.
• the fix to show that even in such situation, the task finally gets resolved into its rightful "cancelled" state.

[oremanj]

This looks good. Please add a newsfragment (newsfragments/161.bugfix.rst) describing this change for the release notes. I just merged #162 which should fix the CI issues once you rebase.

[ngtrunghuan]

This looks good. Please add a newsfragment (newsfragments/161.bugfix.rst) describing this change for the release notes. I just merged #162 which should fix the CI issues once you rebase.

Thanks Josh - I've done both. Please let me know if the formatting on the newsfragment needs work.

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