Fix issue when converting matplotlib lines+markers legend to plotly

[robertoffmoura]

Hi,
I'm proposing a fix to an edge case where conversion from matplotlib to plotly would break when converting a "lines+markers" plot with a legend. The issue happens because go.layout.Shape is not expecting "symbol" or "size" properties. My proposed fix is to ignore those two properties for "lines" type legend shapes. Please let me know if another fix would be more appropriate.

Thanks

Code PR

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