Fix `open_signal_receiver` misannotation

[gschaffner]

Hi!

I believe open_signal_receiver's return type is misannotated. For example:

import signal
from typing import reveal_type

import trio

async def main() -> None:
    with trio.open_signal_receiver(signal.SIGINT, signal.SIGTERM) as signal_receiver:
        async for signum in signal_receiver:
            reveal_type(signum)  # int at runtime, but signal.Signals during type checking!
            print(signum.name)  # passes type checking, but AttributeError at runtime.
            raise SystemExit(1)

if __name__ == "__main__":
    trio.run(main)

[trio-bot[bot]]

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[VincentVanlaer]

Thanks!