Return concrete server from abstract event loop (#5566)

https://docs.python.org/3/library/asyncio-eventloop.html?highlight=abstracteventloop#creating-network-servers
documents the methods as returning a concrete server object.

stdlib/asyncio/events.pyi

@@ -2,15 +2,17 @@ import ssl
 import sys
 from _typeshed import FileDescriptorLike
 from abc import ABCMeta, abstractmethod
-from asyncio.futures import Future
-from asyncio.protocols import BaseProtocol
-from asyncio.tasks import Task
-from asyncio.transports import BaseTransport
-from asyncio.unix_events import AbstractChildWatcher
 from socket import AddressFamily, SocketKind, _Address, _RetAddress, socket
 from typing import IO, Any, Awaitable, Callable, Dict, Generator, List, Optional, Sequence, Tuple, TypeVar, Union, overload
 from typing_extensions import Literal
 
+from .base_events import Server
+from .futures import Future
+from .protocols import BaseProtocol
+from .tasks import Task
+from .transports import BaseTransport
+from .unix_events import AbstractChildWatcher
+
 if sys.version_info >= (3, 7):
     from contextvars import Context
 
@@ -261,7 +263,7 @@ class AbstractEventLoop(metaclass=ABCMeta):
             reuse_port: Optional[bool] = ...,
             ssl_handshake_timeout: Optional[float] = ...,
             start_serving: bool = ...,
-        ) -> AbstractServer: ...
+        ) -> Server: ...
         @overload
         @abstractmethod
         async def create_server(
@@ -279,7 +281,7 @@ class AbstractEventLoop(metaclass=ABCMeta):
             reuse_port: Optional[bool] = ...,
             ssl_handshake_timeout: Optional[float] = ...,
             start_serving: bool = ...,
-        ) -> AbstractServer: ...
+        ) -> Server: ...
         async def create_unix_connection(
             self,
             protocol_factory: _ProtocolFactory,
@@ -300,7 +302,7 @@ class AbstractEventLoop(metaclass=ABCMeta):
             ssl: _SSLContext = ...,
             ssl_handshake_timeout: Optional[float] = ...,
             start_serving: bool = ...,
-        ) -> AbstractServer: ...
+        ) -> Server: ...
         @abstractmethod
         async def sendfile(
             self,
@@ -338,7 +340,7 @@ class AbstractEventLoop(metaclass=ABCMeta):
             ssl: _SSLContext = ...,
             reuse_address: Optional[bool] = ...,
             reuse_port: Optional[bool] = ...,
-        ) -> AbstractServer: ...
+        ) -> Server: ...
         @overload
         @abstractmethod
         async def create_server(
@@ -354,7 +356,7 @@ class AbstractEventLoop(metaclass=ABCMeta):
             ssl: _SSLContext = ...,
             reuse_address: Optional[bool] = ...,
             reuse_port: Optional[bool] = ...,
-        ) -> AbstractServer: ...
+        ) -> Server: ...
         async def create_unix_connection(
             self,
             protocol_factory: _ProtocolFactory,
@@ -372,7 +374,7 @@ class AbstractEventLoop(metaclass=ABCMeta):
             sock: Optional[socket] = ...,
             backlog: int = ...,
             ssl: _SSLContext = ...,
-        ) -> AbstractServer: ...
+        ) -> Server: ...
     @abstractmethod
     async def create_datagram_endpoint(
         self,