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.

从 layout.Legend 入口点开始，检查现有的 update() 行为与字典操作之间的关系。确定 dict API 的哪些部分应受到支持，然后验证所请求的并集操作以及已约定的相关操作是否与 update() 保持一致地运行。