text: Title: Remove f-string for < 3.5 support
  Body: As discovered in https://community.plot.ly/t/jupyterlab-dash-extension/22922, an f-string slipped into the code base which prevents the use of Python 3.5.
  
  https://github.com/plotly/jupyterlab-dash/blob/031a46f527010180b2d2e621e634e260ea545b8b/jupyterlab_dash/__init__.py#L86