ReadOnlySheet doesn't have sheet_state attribute
Created originally on Bitbucket by jeffgao (J. Gao)
I rely on Worksheet.sheet_state to check if a worksheet is hidden in a workbook. For 2.5.x, it worked just the same for workbooks opened in read only mode. However, starting with 2.6.x, ReadOnlyWorksheet is introduced and this class doesn't have sheet_state attribute.
It seems that in 2.6.x if I open a workbook in read only mode, there is no way for me to check if a worksheet is hidden or not.