Fix websocket is not defined error

[tjamet]

With the current version of tydom2mqtt, we can spot this error in the logs:

Traceback (most recent call last):
  File "/app/main.py", line 56, in listen_tydom
    except websockets.ConnectionClosed as e:
           ^^^^^^^^^^
NameError: name websockets is not defined

[tjamet]

duplicate #142