Add type annotation to WebSocketEndpoint.encoding (#1003)

* Add type annotation to WebSocketEndpoint.encoding

* Update endpoints.py

Co-authored-by: Florimond Manca <florimond.manca@gmail.com>

starlette/endpoints.py

@@ -43,7 +43,7 @@ async def method_not_allowed(self, request: Request) -> Response:
 
 class WebSocketEndpoint:
 
-    encoding = None  # May be "text", "bytes", or "json".
+    encoding: typing.Optional[str] = None  # May be "text", "bytes", or "json".
 
     def __init__(self, scope: Scope, receive: Receive, send: Send) -> None:
         assert scope["type"] == "websocket"