expose asyncio.open_unix_connection if it exists (#879)

Also fix the one other place where hasattr() is used (which PEP 484 doesn't support).

stdlib/3.4/asyncio/__init__.pyi

@@ -1,5 +1,6 @@
 """The asyncio package, tracking PEP 3156."""
 
+import socket
 import sys
 from typing import Type
 
@@ -85,6 +86,11 @@ from asyncio.locks import (
 
 if sys.version_info < (3, 5):
     from asyncio.queues import JoinableQueue as JoinableQueue
+if sys.platform != 'win32':
+    from asyncio.streams import (
+        open_unix_connection as open_unix_connection,
+        start_unix_server as start_unix_server,
+    )
 
 # TODO: It should be possible to instantiate these classes, but mypy
 # currently disallows this.

stdlib/3.4/asyncio/streams.pyi

@@ -1,4 +1,4 @@
-import socket
+import sys
 from typing import Any, Awaitable, Callable, Generator, Iterable, Optional, Tuple
 
 from . import coroutines
@@ -38,7 +38,7 @@ def start_server(
     **kwds: Any
 ) -> Generator[Any, None, events.AbstractServer]: ...
 
-if hasattr(socket, 'AF_UNIX'):
+if sys.platform != 'win32':
     @coroutines.coroutine
     def open_unix_connection(
         path: str = ...,