Fixed cancellation retry for tasks with native cancellation

[subotac]

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request being summarily closed without consideration!

Changes

Fixes #1258.

Avoid retrying AnyIO cancellation delivery when the underlying asyncio task already has native cancellation pending. This prevents _must_cancel tasks from being unnecessarily re-armed after leaving a
cancelled scope.

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• Fix big bad boo-boo in task groups
  (#123 <https://github.com/agronholm/anyio/issues/123>_; PR by @yourGithubAccount)

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[agronholm]

Wouldn't this mean that cancellation might not restart for natively cancelled tasks where the target coroutine function caught the cancellation?