`inspect.iscoroutinefunction` returns false on pool -> connection -> execute

asyncpg version: 0.29
PostgreSQL version: 13.11
Do you use a PostgreSQL SaaS? If so, which? Can you reproduce the issue with a local PostgreSQL install?:
Python version: 3.12.2
Platform: linux
Do you use pgbouncer?: No
Did you install asyncpg with pip?: Yes
If you built asyncpg locally, which version of Cython did you use?: -
Can the issue be reproduced under both asyncio and uvloop?: n/a

We noticed that some of our tests clean-up yield RuntimeWarning: coroutine 'Connection.execute' was never awaited when the PYTHONASYNCIODEBUG envvar is set. While investigating, we found that this code in an unittest is the cause of the warning. The root cause is the inspect.iscoroutinefunction on a asyncpg pool connection instance returns False, causing the TestCase class not to await it. https://github.com/python/cpython/blob/16349868d396cc1bff5188de3638321e87fe0293/Lib/unittest/async_case.py#L112-L120

import asyncpg
import inspect
import unittest

class TestAsyncCleanupAwaitsAsyncpgPoolConnectionCoroutine(
    unittest.IsolatedAsyncioTestCase
):
    async def asyncSetUp(self):
        self.pool = await asyncpg.create_pool(min_size=1, max_size=1)
        self.addAsyncCleanup(self.pool.close)
        self.conn = await self.pool.acquire()
        self.addAsyncCleanup(self.pool.release, self.conn)

    async def test_cleanup_awaited(self):
        await self.conn.execute("SELECT now()")
        self.addAsyncCleanup(self.conn.execute, "SELECT now()")
        self.assertTrue(inspect.iscoroutinefunction(self.conn.execute))

class TestAsyncCleanupAwaitsAsyncpgConnectionCoroutine(
    unittest.IsolatedAsyncioTestCase
):
    async def asyncSetUp(self):
        self.conn = await asyncpg.connect()
        self.addAsyncCleanup(self.conn.close)

    async def test_cleanup_awaited(self):
        await self.conn.execute("SELECT now()")
        self.addAsyncCleanup(self.conn.execute, "SELECT now()")
        self.assertTrue(inspect.iscoroutinefunction(self.conn.execute))

if __name__ == "__main__":
    unittest.main()

When running this python snippet with PYTHONASYNCIODEBUG=1 and PYTHONWARNINGS=once and the output is

.F
/home/stj/.local/lib/python3.12/unittest/async_case.py:99: RuntimeWarning: coroutine 'Connection.execute' was never awaited
  self._callMaybeAsync(function, *args, **kwargs)
RuntimeWarning: Enable tracemalloc to get the object allocation traceback

======================================================================
FAIL: test_cleanup_awaited (__main__.TestAsyncCleanupAwaitsAsyncpgPoolConnectionCoroutine.test_cleanup_awaited)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/stj/.local/lib/python3.12/unittest/async_case.py", line 90, in _callTestMethod
    if self._callMaybeAsync(method) is not None:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stj/.local/lib/python3.12/unittest/async_case.py", line 112, in _callMaybeAsync
    return self._asyncioRunner.run(
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stj/.local/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stj/.local/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/home/stj/test_asyncpg.py", line 21, in test_cleanup_awaited
    self.assertTrue(inspect.iscoroutinefunction(self.conn.execute))
AssertionError: False is not true

----------------------------------------------------------------------
Ran 2 tests in 0.060s

FAILED (failures=1)

Expectation is that both tests pass.

MagicStack / asyncpg

`inspect.iscoroutinefunction` returns false on pool -> connection -> execute #1133