Deprecation warning coming from traitlets

[danielhollas]

Discovered when unit testing aiidalab-widgets-base using pytest.

  /opt/hostedtoolcache/Python/3.8.17/x64/lib/python3.8/site-packages/ipywidgets_extended/dropdown.py:37: DeprecationWarning: metadata {'trait': <traitlets.traitlets.Unicode object at 0x7fc9270a29a0>} was set from the constructor. With traitlets 4.1, metadata should be set using the .tag() method, e.g., Int().tag(key1='value1', key2='value2')
    _disabled_options_labels = traitlets.Tuple(