Removed what appears to be dead code since we dont use threads

uvicorn/server.py

@@ -6,7 +6,6 @@
 import signal
 import socket
 import sys
-import threading
 import time
 from email.utils import formatdate
 
@@ -240,12 +239,7 @@ async def shutdown(self, sockets=None):
             await self.lifespan.shutdown()
 
     def install_signal_handlers(self) -> None:
-        if threading.current_thread() is not threading.main_thread():
-            # Signals can only be listened to from the main thread.
-            return
-
         loop = asyncio.get_event_loop()
-
         try:
             for sig in HANDLED_SIGNALS:
                 loop.add_signal_handler(sig, self.handle_exit, sig, None)