Use a str type instead of unicode when searching for the opening '[' in the network packet. Using unicode seems to work, but it really is a byte.

src/calibre/devices/smart_device_app/driver.py

@@ -291,7 +291,7 @@ def _json_encode(self, op, arg):
     def _read_string_from_net(self):
         data = bytes(0)
         while True:
-            dex = data.find('[')
+            dex = data.find(b'[')
             if dex >= 0:
                 break
             # recv seems to return a pointer into some internal buffer.