Replace simple_async_mock with AsyncMock (#16180)

Python 3.8 has a native AsyncMock, use it instead of a custom
implementation.

tests/module_api/test_api.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 from typing import Any, Dict, Optional
-from unittest.mock import Mock
+from unittest.mock import AsyncMock, Mock
 
 from twisted.internet import defer
 from twisted.test.proto_helpers import MemoryReactor
@@ -33,7 +33,6 @@ from synapse.util import Clock
 
 from tests.events.test_presence_router import send_presence_update, sync_presence
 from tests.replication._base import BaseMultiWorkerStreamTestCase
-from tests.test_utils import simple_async_mock
 from tests.test_utils.event_injection import inject_member_event
 from tests.unittest import HomeserverTestCase, override_config
 
@@ -70,7 +69,7 @@ class ModuleApiTestCase(BaseModuleApiTestCase):
     def make_homeserver(self, reactor: MemoryReactor, clock: Clock) -> HomeServer:
         # Mock out the calls over federation.
         self.fed_transport_client = Mock(spec=["send_transaction"])
-        self.fed_transport_client.send_transaction = simple_async_mock({})
+        self.fed_transport_client.send_transaction = AsyncMock(return_value={})
 
         return self.setup_test_homeserver(
             federation_transport_client=self.fed_transport_client,
@@ -579,9 +578,7 @@ class ModuleApiTestCase(BaseModuleApiTestCase):
         """Test that the module API can join a remote room."""
         # Necessary to fake a remote join.
         fake_stream_id = 1
-        mocked_remote_join = simple_async_mock(
-            return_value=("fake-event-id", fake_stream_id)
-        )
+        mocked_remote_join = AsyncMock(return_value=("fake-event-id", fake_stream_id))
         self.hs.get_room_member_handler()._remote_join = mocked_remote_join  # type: ignore[assignment]
         fake_remote_host = f"{self.module_api.server_name}-remote"