Fix remaining instances of shortcut imports.

Author: aaugustin

docs/topics/broadcast.rst

@@ -83,7 +83,7 @@ to::
 
 Here's a coroutine that broadcasts a message to all clients::
 
-    from websockets import ConnectionClosed
+    from websockets.exceptions import ConnectionClosed
 
     async def broadcast(message):
         for websocket in CLIENTS.copy():

docs/topics/keepalive.rst

@@ -112,7 +112,7 @@ You can run a task in the background to send keepalive messages:
     import itertools
     import json
 
-    from websockets import ConnectionClosed
+    from websockets.exceptions import ConnectionClosed
 
     async def keepalive(websocket, ping_interval=30):
         for ping in itertools.count():

experiments/broadcast/server.py

@@ -6,8 +6,8 @@
 import sys
 import time
 
-from websockets import ConnectionClosed
 from websockets.asyncio.server import broadcast, serve
+from websockets.exceptions import ConnectionClosed
 
 
 CLIENTS = set()