Make Listener covariant in the stream type variable

[mila]

This changes allows to properly annotate return type in a function like:

import ssl
from typing import Optional

import anyio
from anyio.abc import ByteStream, Listener
from anyio.streams.tls import TLSListener


async def create_listener(
    local_host: str,
    local_port: int,
    ssl_context: Optional[ssl.SSLContext],
) -> Listener[ByteStream]:
    listener = await anyio.create_tcp_listener(
        local_host=local_host, local_port=local_port
    )
    if ssl_context is not None:
        return TLSListener(listener, ssl_context=ssl_context)
    else:
        return listener

Without this change, the function has to be annotated as returning Union[Listener[SocketStream], TLSListener], which is long and it depends on specific implementation.

The stream type of the listeners acts covariantly. When I have a handler that expects ByteStream, I need Listener[ByteStream]. But the handler won't mind if it gets TLSStream, so I should not mind if I have Listener[TLSStream].

For comparison, callables are covariant in the return type. For the listeners, the socket acts similar as the return value of the callable - it is an output, just passed to the handler callback instead of returned directly.

Note that I did not propagate the change to anyio.streams.stapled.MultiListener. The reason is similar why mutable collections are not covariant. The following function should not accept MultiListener[TLSStream]:

def replace_listeners(multi_listener: MultiListener[ByteStream]):
    multi_listener.listeners = [anyio.create_tcp_listener(...)]

I also did a small change to anyio.abc.SocketListener. Its use of T_Stream seems unnecessary, so I got rid of it instead of replacing it by T_Stream_co.

[mila]

The fail of pre-commit.ci is unrelated, so I addressed it in #465.

[agronholm]

Would you mind fixing the conflicts? I recently merged the 4.0-dev branch to master in preparation for a major release. I would like this change to be a part of that release.

[agronholm]

Checked with mypy, LGTM.

[agronholm]

Thanks!

[mila]

Thank you for merging.

BTW I am looking forward to the 4.0 release. I looked at the changelog today and found that at least three of the changes will help me somehow.