fix(src): Close connections to API clients when shutting down

This effectively re-implements asyncio.Server::close_clients which
is only available in python3.13.

In python3.12 asyncio.Server::wait_closed will hang unless these
sockets are closed: python/cpython#104344

src/wsdd.py

@@ -1127,6 +1127,7 @@ class ApiServer:
     def __init__(self, aio_loop: asyncio.AbstractEventLoop, listen_address: bytes,
                  address_monitor: 'NetworkAddressMonitor') -> None:
         self.server = None
+        self.clients = set()
         self.address_monitor = address_monitor
 
         # defer server creation
@@ -1146,6 +1147,7 @@ async def create_server(self, aio_loop: asyncio.AbstractEventLoop, listen_addres
                 self.on_connect, path=listen_address))
 
     async def on_connect(self, read_stream: asyncio.StreamReader, write_stream: asyncio.StreamWriter) -> None:
+        self.clients.add(write_stream.transport)
         while True:
             try:
                 line = await read_stream.readline()
@@ -1219,6 +1221,8 @@ async def cleanup(self) -> None:
         await self.create_task
         if self.server:
             self.server.close()
+            for transport in self.clients:
+                transport.close()
             await self.server.wait_closed()