ws.columns has exception if the workbook is loaded with read_only=True
*Created originally on Bitbucket by [StevenNiu (Steven Niu)](https://bitbucket.org/%7B10bc1499-f089-499a-914d-b00234ff7fc9%7D/)* ``` #!python In [47]: wb = load_workbook('t2.xlsx', read_only=True) In [48]: ws = wb.get_sheet_by_name(wb.sheetnames[0]) In [49]: ws.rows Out[49]: <generator object get_squared_range at 0x0000000015D4DF30> In [50]: ws.columns Traceback (most recent call last): File "<ipython-input-50-17e09f4be84e>", line 1, in <module> ws.columns File "C:\Users\Steven\Anaconda\lib\site-packages\openpyxl\worksheet\read_only.py", line 189, in columns return super(IterableWorksheet, self).columns NameError: global name 'IterableWorksheet' is not defined ```
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