Fix _FakeServer to immediately close client connections

Without this, if the SMTP server throws an exception during
initialization, Controller.stop() gets stuck indefinitely waiting
on active connections.

Note: this only happens in Python 3.12+. Earlier versions of Python
allowed `wait_closed()` to complete regardless of active connections.

With this change the TestFactory testcases can again be enabled for
Python 3.12+.

Fixes: #394

Author: cuu508

aiosmtpd/controller.py

@@ -108,6 +108,9 @@ def _cb_client_connected(
     ) -> None:
         pass
 
+    def connection_made(self, transport):
+        transport.close()
+
 
 @public
 class BaseController(metaclass=ABCMeta):

aiosmtpd/tests/test_server.py

@@ -535,7 +535,6 @@ def test_inet_contstop(self, temp_event_loop, runner):
         assert temp_event_loop.is_closed() is False
 
 
-@pytest.mark.skipif(sys.version_info >= (3, 12), reason="Hangs on 3.12")
 @pytest.mark.filterwarnings("ignore::pytest.PytestUnraisableExceptionWarning")
 class TestFactory:
     def test_normal_situation(self):