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
```
issue