Streamline-plots examples out of date after v 3.0.0

[jonmmease]

Originally opened by @PiotrBenedysiuk in https://github.com/plotly/plotly.py/issues/1282:

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The second plot of streamline plots doesn't run on pyplot v3.4.1. The traceback reads:

AttributeError Traceback (most recent call last) in () 31 32 # Add source point to figure ---> 33 fig['data'].append(source_point) 34 py.iplot(fig, filename='streamline_source')

AttributeError: 'tuple' object has no attribute 'append'

I think its due to fig[data] being represented as a tuple from 3.0 onwards. Please update the example to use fig.add_trace instead.