fix-python-websockets#762: for consistency, call ensure_future for py36

Author: DavidBord

src/websockets/server.py

@@ -715,8 +715,8 @@ async def _close(self) -> None:
         if self.websockets:
             await asyncio.wait(
                 [
-                    websocket.close(1001)
-                    if sys.version_info[:2] < (3, 8)
+                    asyncio.ensure_future(websocket.close(1001))
+                    if sys.version_info[:2] < (3, 7)
                     else asyncio.create_task(websocket.close(1001))
                     for websocket in self.websockets
                 ],