asyncio protocol factory types are too broad #6936

Here's an example:

import asyncio


class MyProtocol(asyncio.Protocol):
    def hello(self) -> None:
        pass


async def f() -> None:
    _, protocol = await asyncio.get_event_loop().create_connection(MyProtocol)
    protocol.hello()

Mypy will say that error: "BaseProtocol" has no attribute "hello". That's because current stubs say that returned protocol type will be BaseProtocol. But we know for sure it will be the same as a type of the value returned by protocol factory (first argument).

It could be made like this:

_ProtocolT = TypeVar("_ProtocolT", bound=BaseProtocol)
async def create_connection(self, protocol_factory: Callable[[], _ProtocolT], ...) -> tuple[BaseTransport, _ProtocolT]:

This way type checking will be more precise and there will be no need for casts. What to you think?