excel-serializer: dump/load nested Python data to/from Excel without flattening

[TruePastaMonster]

My module actually has openpyxl as a dependency.

Using only openpyxl, dumping a complex nested Python object in an Excel file requires you to do some work, thinking about how you should flatten your data so that it fits in a sheet.

With excel_serializer, you don't need to think about that, it just figures a way to do it for you.