ENH: Implement _Openpyxl2Writer for pandas.io.excel

This patch is an implementation of a ``pandas.io.excel.ExcelWriter`` subclass that is compatible with the style interface of ``openpyxl`` 2.0.0 or later.

closes #7177
You mean a `cell` that the `cells` argument to the `ExcelWriter.write_cells()` method would yield? All of the writers must handle `cell.style` conforming to a certain structure, which, though not documented in the `ExcelWriter.write_cells()` doc string, can be inferred from `core.format.header_style`, `core.format.ExcelCell`, and various methods of `core.format.ExcelFormatter`, not to mention direct inspection of the concrete implementations of `ExcelWriter.write_cells()`.

Are you asking why `_OpenpyxlWriter` subclasses `_OpenpyxlWriter1`, and maybe suggesting that it shouldn't?

I feel like I'm missing the point of your question.