cassandra/cluster.py: make asyncio default if asyncore not available

since python 3.12 is deprecating asyncore, we should make
asyncio the default fallback event loop when asyncore isn't available

asyncio now that it's fixed and we verified it's working (passing the integration suite)
in multiple python versions we support (from 3.8 - 3.12)

cassandra/cluster.py

@@ -139,7 +139,10 @@ def _is_gevent_monkey_patched():
     try:
         from cassandra.io.libevreactor import LibevConnection as DefaultConnection  # NOQA
     except ImportError:
-        from cassandra.io.asyncorereactor import AsyncoreConnection as DefaultConnection  # NOQA
+        try:
+            from cassandra.io.asyncorereactor import AsyncoreConnection as DefaultConnection # NOQA
+        except ImportError:
+            from cassandra.io.asyncioreactor import AsyncioConnection as DefaultConnection  # NOQA
 
 # Forces load of utf8 encoding module to avoid deadlock that occurs
 # if code that is being imported tries to import the module in a seperate