ruff found issues

trio/_tests/test_highlevel_serve_listeners.py:(40:15 - 40:46): "Tuple[MemorySendChannel[StapledStream[MemorySendStream, MemoryReceiveStream]], MemoryReceiveChannel[StapledStream[MemorySendStream, MemoryReceiveStream]]]" has no attribute "send_channel" [attr-defined]

trio/_tests/test_highlevel_serve_listeners.py:(48:24 - 48:58): "Tuple[MemorySendChannel[StapledStream[MemorySendStream, MemoryReceiveStream]], MemoryReceiveChannel[StapledStream[MemorySendStream, MemoryReceiveStream]]]" has no attribute "receive_channel" [attr-defined]

trio/_tests/test_highlevel_serve_listeners.py:(50:9 - 50:21): Returning Any from function declared to return "StapledStream[MemorySendStream, MemoryReceiveStream]" [no-any-return]

trio/_tests/test_channel.py:(418:1 - 420:58): Function is missing a return type annotation [no-untyped-def]

trio/_tests/test_channel.py:(419:12 - 419:33): Need type annotation for "pair" [var-annotated]

Mypy found type errors.

Type completeness changed, please update!

Process completed with exit code 1.