read only worksheet raises DeprecationWarning
*Created originally on Bitbucket by [scott_stanton_veeva (Scott Stanton)](https://bitbucket.org/%7B7a9586cf-c859-4dd6-94a3-64b2af8141ac%7D/)*
A simple access to a cell via a read-only workbook results in a deprecation warning because get_squared_range is still used by ReadOnlyWorksheet._get_cell():
```
#!python
>>> wb=load_workbook('myfile.xlsx', true)
>>> ws=wb['Employee List']
>>> ws['A1']
openpyxl/worksheet/read_only.py:189: DeprecationWarning: Call to deprecated function get_squared_range (Use ws.iter_rows()).
for row in self.get_squared_range(column, row, column, row):
<ReadOnlyCell 'Employee List'.A1>
>>>
```
issue