Fix reference to event loop in serve_forever method

[DVNghiem]

This pull request includes a minor change to the serve_forever method in rloop/server.py. The change replaces the use of self._loop.create_future() with self.get_loop().create_future() to ensure compatibility with the loop retrieval method.

import asyncio
import rloop
import traceback

async def start_server(host: str, port: int):
    server = await asyncio.start_server(
        lambda r, w: asyncio.Protocol().connection_made(w), host, port
    )
    try:
        async with server:
            await server.serve_forever()
    except Exception as e:
        traceback.print_exc()

if __name__ == "__main__":
    loop = rloop.new_event_loop()
    loop.run_until_complete(start_server("0.0.0.0", 8888))

Exception is:
line 30, in serve_forever self._sff = self._loop.create_future() ^^^^^^^^^^ AttributeError: 'Server' object has no attribute '_loop'

[gi0baro]

Thank you for your contribution ❤️
I'd say even using the "private" interface (self._inner._loop) is fine in this case.

[DVNghiem]

Thank you so much! I will update accordingly.