gh-84753: Make inspect.iscoroutinefunction() work with AsyncMock (#94050)

The inspect version was not working with unittest.mock.AsyncMock.

The fix introduces special-casing of AsyncMock in
`inspect.iscoroutinefunction` equivalent to the one
performed in `asyncio.iscoroutinefunction`.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/test/test_asyncio/test_tasks.py

@@ -1649,6 +1649,7 @@ class BaseTaskTests:
         self.assertTrue(asyncio.iscoroutinefunction(fn2))
 
         self.assertFalse(asyncio.iscoroutinefunction(mock.Mock()))
+        self.assertTrue(asyncio.iscoroutinefunction(mock.AsyncMock()))
 
     def test_coroutine_non_gen_function(self):
         async def func():