Fixed cancellation retry for tasks with native cancellation

[subotac]

NOTE Erasing or replacing the contents of this template will result in your pull
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.

Checklist

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you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (in docs/), in case of behavior changes or new
  features
• You've added a new changelog entry (in docs/versionhistory.rst).

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Updating the changelog

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If, say, your patch fixes issue #123, the entry should look like this:

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

If there's no issue linked, just link to your pull request instead by updating the
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