Use more precise return types in asyncio methods

[drtyrsa]

No description provided.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

websockets (https://github.com/aaugustin/websockets)
+ src/websockets/legacy/client.py:641: error: "_ProtocolT" has no attribute "close"
+ src/websockets/legacy/client.py:655: error: Incompatible types in assignment (expression has type "WebSocketClientProtocol", variable has type "_ProtocolT")
+ src/websockets/legacy/client.py:659: error: "_ProtocolT" has no attribute "handshake"
+ src/websockets/legacy/client.py:661: error: "_ProtocolT" has no attribute "origin"
+ src/websockets/legacy/client.py:662: error: "_ProtocolT" has no attribute "available_extensions"
+ src/websockets/legacy/client.py:663: error: "_ProtocolT" has no attribute "available_subprotocols"
+ src/websockets/legacy/client.py:664: error: "_ProtocolT" has no attribute "extra_headers"
+ src/websockets/legacy/client.py:667: error: "_ProtocolT" has no attribute "fail_connection"
+ src/websockets/legacy/client.py:668: error: "_ProtocolT" has no attribute "wait_closed"
+ src/websockets/legacy/client.py:672: error: Incompatible return value type (got "_ProtocolT", expected "WebSocketClientProtocol")

anyio (https://github.com/agronholm/anyio)
+ src/anyio/_backends/_asyncio.py:1365: error: Redundant cast to "StreamProtocol"  [redundant-cast]
+ src/anyio/_backends/_asyncio.py:1555: error: Redundant cast to "DatagramProtocol"  [redundant-cast]

aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/connector.py:1295: error: Redundant cast to "ResponseHandler"  [redundant-cast]

[srittau]

LGTM, the anyio and aiohttp primer output looks promising. websockets is doing some complicated stuff when creating protocols, so I think they'd just need to update their annotations. I'd like a second opinion, though.

[Akuli]

websockets does this:

        if kwargs.pop("unix", False):
            ...
            create_connection = functools.partial(
                loop.create_unix_connection, factory, path, **kwargs
            )
        else:
            ...
            create_connection = functools.partial(
                loop.create_connection, factory, host, port, **kwargs
            )
        ...
        self._create_connection = create_connection

Code like this isn't very typing friendly, and I don't think there's a good way for us to support it anyway. Websockets already has casts.