Drop asynctest (#44746)

tests/util/test_async.py

@@ -1,13 +1,12 @@
 """Tests for async util methods from Python source."""
 import asyncio
 import time
+from unittest.mock import MagicMock, Mock, patch
 
 import pytest
 
 from homeassistant.util import async_ as hasync
 
-from tests.async_mock import MagicMock, Mock, patch
-
 
 @patch("asyncio.coroutines.iscoroutine")
 @patch("concurrent.futures.Future")