Missing XML Metadata in workbook.xml

[jhramey]

When using the latest version of openpyxl, it appears we are missing some metadata that can be found when creating a spreadsheet using Excel
mc:Ignorable="x15" xmlns:x15="http://schemas.microsoft.com/office/spreadsheetml/2010/11/main"

I understand the spreadsheet still works without this metadata but if you open the spreadsheet in excel and resave the file, this gets added anyways. Would be nice if the xml/constants.py was updated to add the above metadata.