Fix streaming news article event dispatching.

News article events are different from other types of events (e.g. stock trades) because a single news article could be applicable to multiple symbols. For that reason, the streamed event does not have the usual `S` field that contains a single symbol; instead, the event has a `symbols` field that contains a list of symbols.

alpacahq#555 mistakenly assumes that the event has an `S` field. This commit changes the dispatching logic to instead use the `symbols` field.

(There is a similar [commit](alpacahq/alpaca-trade-api-csharp@be67b62) for the C# library.)

alpaca_trade_api/stream.py

@@ -565,12 +565,20 @@ def _cast(self, msg_type, msg):
 
     async def _dispatch(self, msg):
         msg_type = msg.get('T')
-        symbol = msg.get('S')
         if msg_type == 'n':
-            handler = self._handlers['news'].get(
-                symbol, self._handlers['news'].get('*', None))
-            if handler:
-                await handler(self._cast(msg_type, msg))
+            symbols = msg.get('symbols', [])
+            # A news article could be unrelated to any symbols, resulting in an empty symbols list. Those news articles
+            # should still be dispatched to the wildcard event handler.
+            if not symbols:
+                symbols.append('*')
+
+            for symbol in symbols:
+                handler = self._handlers['news'].get(symbol)
+                if handler is None:
+                    handler = self._handlers['news'].get('*')
+
+                if handler:
+                    await handler(self._cast(msg_type, msg))
         else:
             await super()._dispatch(msg)