Get rid of deprecation warning emitted by Python 3.8, which causes a test failure(!).

https://github.com/bitcoin/bitcoin/pull/17931

Taken from upstream:

https://github.com/bitcoin/bitcoin/commit/f117fb00da747147cddfb071c1427a2754c278cd

diff --git a/test/functional/p2p_invalid_messages.py b/test/functional/p2p_invalid_messages.py
index 20864881c165..07eacf410d88 100755
--- a/test/functional/p2p_invalid_messages.py
+++ b/test/functional/p2p_invalid_messages.py
@@ -145,13 +145,13 @@ def run_test(self):
     def test_magic_bytes(self):
         conn = self.nodes[0].add_p2p_connection(P2PDataStore())
 
-        def swap_magic_bytes():
+        async def swap_magic_bytes():
             conn._on_data = lambda: None  # Need to ignore all incoming messages from now, since they come with "invalid" magic bytes
             conn.magic_bytes = b'\x00\x11\x22\x32'
 
         # Call .result() to block until the atomic swap is complete, otherwise
         # we might run into races later on
-        asyncio.run_coroutine_threadsafe(asyncio.coroutine(swap_magic_bytes)(), NetworkThread.network_event_loop).result()
+        asyncio.run_coroutine_threadsafe(swap_magic_bytes(), NetworkThread.network_event_loop).result()
 
         with self.nodes[0].assert_debug_log(['PROCESSMESSAGE: INVALID MESSAGESTART ping']):
             conn.send_message(messages.msg_ping(nonce=0xff))