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.

從 layout.Legend 進入點開始，檢查現有的 update() 行為與字典操作之間的關係。確定 dict API 的哪些部分應受到支援，然後驗證所要求的聯集操作以及已議定的相關操作是否與 update() 一致地運作。