Replace asynctest with stdlib mock

The asynctest package is unmaintained since 2020 and incompatible with
Python 3.12+. Replace asynctest.TestSelector with a MagicMock using
selectors.BaseSelector as its spec, which provides the same mock
selector functionality needed by the asyncio reactor tests.

Fixes #663

Author: dkropachev

pyproject.toml

@@ -52,7 +52,6 @@ dev = [
     "cython>=3.2",
     "packaging>=25.0",
     "futurist",
-    "asynctest",
     "pyyaml",
     "numpy",
     "objgraph",

tests/unit/io/test_asyncioreactor.py

@@ -1,7 +1,6 @@
 AsyncioConnection, ASYNCIO_AVAILABLE = None, False
 try:
     from cassandra.io.asyncioreactor import AsyncioConnection
-    import asynctest
     ASYNCIO_AVAILABLE = True
 except (ImportError, SyntaxError, AttributeError):
     AsyncioConnection = None
@@ -10,8 +9,8 @@
 from tests import is_monkey_patched, connection_class
 from tests.unit.io.utils import TimerCallback, TimerTestMixin
 
-from unittest.mock import patch
-
+from unittest.mock import patch, MagicMock
+import selectors
 import unittest
 import time
 
@@ -56,7 +55,7 @@ def setUp(self):
         socket_patcher.start()
 
         old_selector = AsyncioConnection._loop._selector
-        AsyncioConnection._loop._selector = asynctest.TestSelector()
+        AsyncioConnection._loop._selector = MagicMock(spec=selectors.BaseSelector)
 
         def reset_selector():
             AsyncioConnection._loop._selector = old_selector