Allow range selector to work for sub-second steps

I commonly have time series dataframes with sub-second frequencies (milli/micro/nano) and I would like to use Scatter plots with rangeselector to visualize them. Issue is rangeselector's buttons can only be provided with a step included in the following:

['month', 'year', 'day', 'hour', 'minute', 'second', 'all']

I would like to be able to pass as well: nanosecond, microsecond, and millisecond.

I am not familiar with the plotly.py codebase but I am happy to open a PR provided I can have some guidance.

首先跟踪 plotly.py 如何验证或序列化 Scatter 图的 rangeselector 按钮，重点关注 issue 中所述的允许的 step 值。通过使用 nanosecond、microsecond 和 millisecond step 测试 rangeselector，并验证生成的绘图配置被接受，来确认这一更改。