Allow callback= to be an awaitable function

[simonw]

If callback= could be provided an awaitable, users could construct functions that e.g. make an async call to a database as part of deciding if a host should be allowed.

I tried implementing this with a asyncio.isawaitable() test against the callable but for some reason it didn't detect my await function as being awaitable. Here's the unit test I added which I failed to get passing:

@pytest.mark.asyncio
async def test_callback_async():
    was_called = False

    async def callback_true(origin):
        return True

    async def callback_false(origin):
        return False

    assert EXAMPLE_HOST == await get_cors_header(asgi_cors(hello_world_app, callback=callback_true), EXAMPLE_HOST)
    assert None == await get_cors_header(asgi_cors(hello_world_app, callback=callback_false), EXAMPLE_HOST)

[dmgolembiowski]

@simonw Admittedly, this might not be much help to you (or relevant anymore) but would changing asyncio.iscoroutine(...) to asyncio.iscoroutinefunction(...) fix the issue?