Camera: replace deprecated @web.asynchronous with async def

Tornado removed @web.asynchronous in 6.x; bbctrl on the Pi runs an
older but compatible async-aware build. Switching to coroutine syntax
keeps the streaming endpoint working across Tornado 5/6.

src/py/bbctrl/Camera.py

@@ -468,8 +468,7 @@ class VideoHandler(web.RequestHandler):
         self.camera = app.camera
 
 
-    @web.asynchronous
-    def get(self):
+    async def get(self):
         self.request.connection.stream.max_write_buffer_size = 10000
 
         self.set_header('Cache-Control', 'no-store, no-cache, must-revalidate, '