COMPAT: Updated method call for _Openpyxl22Write.write_cells() for Openpyxl =>2.4.0 #14519
- [X] closes #14519
- [X] tests added / passed
- [X] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Updated _Openpyxl22Writer.write_cells() to use new coordinate call method wks['coordinate']
instead of wks.cell() in accordance with https://openpyxl.readthedocs.io/en/default/tutorial.html#accessing-one-cell
in the newest release of Openpyxl (2.4.0 and later).

I did not know how to write a true test for this fix since it involves Excel file I/O and pandas does not currently supporting reading in .xlsx as far as I know. The best I could do is use it with openpyxl 2.4.0 install on my machine and passing a small dataframe into an .xlsx. Please let me know if you have any suggestions on how to better test this. 
