[typing] Fix return annotation of __aexit__

[Enegg]

Changes

A follow-up to #849. I had done a search for def __exit__ in the prior PR, forgetting that __aexit__ is also a thing.

I tried adding the following "test" (static assert, not supposed to be run), but what works with pyright fails with mypy ¯\_(ツ)_/¯

from typing import assert_type

async def assert_context_manager_reachability() -> None:
    # typing-only check
    tg_var = ()

    async with create_task_group():
        tg_var = int("123")

    assert_type(tg_var, "int | tuple[()]")

    cs_var = ()
    with CancelScope():  # noqa: ASYNC100
        cs_var = int("123")

    assert_type(cs_var, "int | tuple[()]")

Checklist

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• You've added tests (in tests/) added 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)

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