Test failure in test_pool_handles_transaction_exit_in_asyncgen_2 with Python 3.15.0a5, PYTHONASYNCIODEBUG=1

$ git clone https://github.com/MagicStack/asyncpg.git
$ cd asyncpg
$ git submodule update --init --recursive
$ uv venv --python 3.15
Using CPython 3.15.0a5 interpreter at: /usr/bin/python3.15
Creating virtual environment at: .venv
Activate with: source .venv/bin/activate
$ uv pip install -e .[gssauth]
$ uv pip install --group test
$ PYTHONASYNCIODEBUG=1 pytest -v
[…]
________________________________________________________________ TestPool.test_pool_handles_transaction_exit_in_asyncgen_2 _________________________________________________________________
Traceback (most recent call last):
  File "/usr/lib64/python3.15/unittest/case.py", line 58, in testPartExecutor
    yield
  File "/usr/lib64/python3.15/unittest/case.py", line 667, in run
    self._callTestMethod(testMethod)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/usr/lib64/python3.15/unittest/case.py", line 613, in _callTestMethod
    result = method()
  File "/home/ben/src/forks/asyncpg/asyncpg/_testbase/__init__.py", line 92, in wrapper
    self.loop.run_until_complete(coro)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/usr/lib64/python3.15/asyncio/base_events.py", line 719, in run_until_complete
    return future.result()
           ~~~~~~~~~~~~~^^
RuntimeError: cannot reuse already awaited coroutine

The above instructions also result in TestConnectionAttributes.test_prefer_standby_picks_master_when_standby_is_down failing due to a timeout, but I’m not seeing that when I build https://src.fedoraproject.org/rpms/python-asyncpg.

The test TestPool.test_pool_handles_transaction_exit_in_asyncgen_2 passes when PYTHONASYNCIODEBUG=1 is not set.

使用 Python 3.15.0a5、PYTHONASYNCIODEBUG=1 和文档中记录的 pytest 命令重现该故障。先从 TestPool.test_pool_handles_transaction_exit_in_asyncgen_2 和 asyncpg/_testbase/init.py:92 开始，然后比较不启用 asyncio 调试时的运行结果，并检查单独的 standby 超时；完成的标准是已理解并解决报告的测试失败，或已将其明确隔离。