ENH: Rename ExcelWriter to make clear attributes are not public
Ref: https://github.com/pandas-dev/pandas/issues/43068#issuecomment-900504112

Copying from the comment above:

> The [documentation](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.ExcelWriter.html) of `ExcelWriter` says: "None of the methods and properties are considered public.".

It seems this line the documentation would be easy to miss, and it'd be better to make this more clear via the standard naming convention of a single leading underscore.