Set stack level and explicit warning type

Author: jacobtomlinson

distributed/core.py

@@ -731,7 +731,9 @@ async def listen(self, port_or_addr=None, allow_offload=True, **kwargs):
                 raise
             warnings.warn(
                 f"Address {addr} is already in use.\n"
-                f"Falling back to {fallback_port_or_addr} instead"
+                f"Falling back to {fallback_port_or_addr} instead",
+                UserWarning,
+                stacklevel=2,
             )
             listener = await listen(
                 fallback_port_or_addr,