title: Openpyxl fails to read row when reference attribute is missing
body: <p>I'm using openpyxl to read an excel worksheet. It throws a KeyError when iterating and parsing rows because the openxml row element doesn't have a reference attribute ('r'). It only has a style index ('s') and 'customFormat'.</p>
  
  <p><a href="https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.cell?view=openxml-2.8.1#remarks" rel="nofollow noreferrer">Openxml spreadsheet attributes</a></p>
  
  <p>How can I read the cells in this file?</p>
  
  <p>The error occurs whether the workbook is opened as read_only or not, and when referencing cells different ways because they all call <code>openpyxl.worksheet._reader.parse_row()</code></p>
  
  <p>I can't share the excel workbook, but the python code is very simple.</p>
  
  <pre><code>wb = load_workbook(filename)
  ws = wb['Sheet1']
  print(ws['A1'])
  </code></pre>
  
  <p>The error:</p>
  
  <pre><code>  File "openpyxl\worksheet\_reader.py", line 150, in parse
      row = self.parse_row(element)
    File "openpyxl\worksheet\_reader.py", line 272, in parse_row
      self.row_dimensions[attrs['r']] = attrs
  KeyError: 'r'
  </code></pre>
  
  <p>From the debugger:</p>
  
  <pre><code>row = {Element} &lt;Element '{http://schemas.openxmlformats.org/spreadsheetml/2006/main}row' at 0x000002713782E9F8&gt;
      attrib = {dict} &lt;class 'dict'&gt;: {'s': '148', 'customFormat': '1'}
      tag = {str} '{http://schemas.openxmlformats.org/spreadsheetml/2006/main}row'
      tail = {NoneType} None
      text = {NoneType} None
  </code></pre>