ReadOnlyWorksheet.cell() raises an error.
*Created originally on Bitbucket by [slexbi (Alexander)](https://bitbucket.org/%7Beb984bc1-9273-47d5-9012-8a9a6ba35a5b%7D/)*
```
#!python
wb0 = load_workbook('/home/myname/myfile.xlsx', read_only=True)
ws0 = wb0['Sheet1']
ws0.cell(1, 1)
TypeError: unbound method cell() must be called with Worksheet instance as first argument (got int instance instead)
ws0.cell(ws0, 1, 1)
TypeError: unbound method cell() must be called with Worksheet instance as first argument (got ReadOnlyWorksheet instance instead)
```
issue