text: Title: ipyleaflet map is not rendered when ruhning in papermill
  Body: I have the following cell:
  
  ```
  from IPython.display import display
  from ipyleaflet import Map, Polyline
  
  m = Map(center=(32.2, 34.8), zoom=12)
  for r in df.itertuples():
      line = Polyline(
          locations=[[[r.start_latitude, r.start_longitude], [r.end_latitude, r.end_longitude]]],
          weight=15
      )
      m.add_layer(line)
  
  display(m)
  ```
  
  The map is displayed correctly, when running interactively. However, when running the same notebook using papermill it renders just this:
  
  ```
  Map(basemap={'url': 'https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', 'max_zoom': 19, 'attribution': 'Map …
  ```