BUG: AttributeError: 'Cell' object has no attribute 'font' when using to_excel() with openpyxl engine
- [x]  I have checked that this issue has not already been reported.
- [x] I have confirmed this bug exists on the latest version of pandas.
- [x] I have confirmed this bug exists on the latest version of openpyxl.
- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.


I am using to_excel() with openpyxl engine to export a pandas dataframe to a sheet of an .xlsx file. My code works just fine two days ago, but it does not work now with Error:
```
with pd.ExcelWriter(PATH+ '/' + FILENAME, engine='openpyxl', mode='a') as writer:
      df.head().to_excel(writer, sheet_name='test')
      writer.save()



---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-69-1e1ecfd20f72> in <module>
      1 with pd.ExcelWriter(PATH+ '/' + FILENAME, engine='openpyxl', mode='a') as writer:
----> 2     df.head().to_excel(writer, sheet_name='test')
      3     writer.save()
      4 
      5 writer.close()

/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pandas/core/generic.py in to_excel(self, excel_writer, sheet_name, na_rep, float_format, columns, header, index, index_label, startrow, startcol, engine, merge_cells, encoding, inf_rep, verbose, freeze_panes)
   2027             startcol=startcol,
   2028             freeze_panes=freeze_panes,
-> 2029             engine=engine,
   2030         )
   2031 

/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pandas/io/formats/excel.py in write(self, writer, sheet_name, startrow, startcol, freeze_panes, engine)
    737             startrow=startrow,
    738             startcol=startcol,
--> 739             freeze_panes=freeze_panes,
    740         )
    741         if need_save:

/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pandas/io/excel/_openpyxl.py in write_cells(self, cells, sheet_name, startrow, startcol, freeze_panes)
    437             if style_kwargs:
    438                 for k, v in style_kwargs.items():
--> 439                     setattr(xcell, k, v)
    440 
    441             if cell.mergestart is not None and cell.mergeend is not None:

AttributeError: 'Cell' object has no attribute 'font'









```