Log Message:
py-openpyxl: updated to 2.6.2

2.6.2:
Bugfixes
* Workbook has no _date_formats attribute
* Cannot create charts for worksheets with quotes in the title
* MergedCells not removed when range is unmerged
* Link to pivot table lost from charts
* Chart colours change after saving
* Cannot use ws.cell in read-only mode with Python 2.7