use explicit utf-8 encoding when opening files in text mode

lib/websockets.py

@@ -64,7 +64,7 @@ class WsClientThread(util.DaemonThread):
         # read json file
         rdir = self.config.get('requests_dir')
         n = os.path.join(rdir, 'req', request_id[0], request_id[1], request_id, request_id + '.json')
-        with open(n) as f:
+        with open(n, encoding='utf-8') as f:
             s = f.read()
         d = json.loads(s)
         addr = d.get('address')