Bug: load_work do not work in some Excel files

Summary

• Older version excel file cannot be read in version 3.0.6.

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-60a6b3beef1a> in <module>
----> 1 wb = openpyxl.load_workbook("hoge.xlsx")

~/dir/.venv/lib/python3.8/site-packages/openpyxl/reader/excel.py in load_workbook(filename, read_only, keep_vba, data_only, keep_links)
    315     reader = ExcelReader(filename, read_only, keep_vba,
    316                         data_only, keep_links)
--> 317     reader.read()
    318     return reader.wb

~/dir/.venv/lib/python3.8/site-packages/openpyxl/reader/excel.py in read(self)
    280         self.read_theme()
    281         apply_stylesheet(self.archive, self.wb)
--> 282         self.read_worksheets()
    283         self.parser.assign_names()
    284         if not self.read_only:

~/dir/.venv/lib/python3.8/site-packages/openpyxl/reader/excel.py in read_worksheets(self)
    226                 ws._rels = rels
    227                 ws_parser = WorksheetReader(ws, fh, self.shared_strings, self.data_only)
--> 228                 ws_parser.bind_all()
    229 
    230             # assign any comments to cells

~/dir/.venv/lib/python3.8/site-packages/openpyxl/worksheet/_reader.py in bind_all(self)
    441     def bind_all(self):
    442         self.bind_cells()
--> 443         self.bind_merged_cells()
    444         self.bind_hyperlinks()
    445         self.bind_formatting()

~/dir/.venv/lib/python3.8/site-packages/openpyxl/worksheet/_reader.py in bind_merged_cells(self)
    378         for cr in self.parser.merged_cells.mergeCell:
    379             mcr = MergedCellRange(self.ws, cr.ref)
--> 380             self.ws._clean_merge_range(mcr)
    381             ranges.append(mcr)
    382         self.ws.merged_cells = MultiCellRange(ranges)

~/dir/.venv/lib/python3.8/site-packages/openpyxl/worksheet/worksheet.py in _clean_merge_range(self, mcr)
    605         for row, col in cells:
    606             self._cells[row, col] = MergedCell(self, row, col)
--> 607         mcr.format()
    608 
    609 

~/dir/.venv/lib/python3.8/site-packages/openpyxl/worksheet/merge.py in format(self)
    107         for name in names:
    108             side = getattr(self.start_cell.border, name)
--> 109             if side.style is None:
    110                 continue # don't need to do anything if there is no border style
    111             border = Border(**{name:side})

AttributeError: 'NoneType' object has no attribute 'style'

• This incident does not occur in version 3.0.3
• I haven't yet confirmed which Excel environment the problem occurs in the saved files.