Add private attrs to `PlotlyList` and `PlotlyDict`
Add private attrs to `PlotlyList` and `PlotlyDict`

`_name` refers to the object name (‘scatter’)
`_items` is a set of items that can go in the object (‘annotation’
    in ‘annotations’)
`_attributres` is a set of strings that can be used as attributes for
    the object (‘width’ for ‘line’)
`_parent_key` is the key that this object is defined under. Usually a
    bit redundant, but `textfont` for `Font` is a good example.
plotly/plotly.py,plotly/python-api,plotly/python-api,plotly/python-api,plotly/plotly.py,plotly/plotly.py