Support dict union operator in legend updates

Would be nice if layout.Legend instances supported the dict union operator:

import plotly.graph_objects as go

fig = go.Figure()

fig.layout.legend |= dict(title="")

TypeError: unsupported operand type(s) for |=: 'Legend' and 'dict'

Since legend.update() works in analogy to dict.update()

fig.layout.legend.update(dict(title=""))

>>> layout.Legend({
    'title': {'text': ''}
})

I was expecting other parts of the dict API to work as well.

Start at the layout.Legend entry point and inspect how its existing update() behavior relates to dictionary operations. Determine which parts of the dict API are intended to be supported, then verify that the requested union operation and the agreed related operations behave consistently with update().