title: Python openpyxl data_only=True returning None
body: <p>I have a simple excel file:</p>
  
  <pre><code>A1 = 200
  A2 = 300
  A3 = =SUM(A1:A2)
  </code></pre>
  
  <p>this file works in excel and shows proper value for <code>SUM</code>, but while using openpyxl module for python I cannot get value in <code>data_only=True</code> mode</p>
  
  <p>Python code from shell:</p>
  
  <pre class="lang-py prettyprint-override"><code>wb = openpyxl.load_workbook('writeFormula.xlsx', data_only = True)
  sheet = wb.active
  sheet['A3']
  &lt;Cell Sheet.A3&gt; # python response
  print(sheet['A3'].value)
  None # python response
  </code></pre>
  
  <p>while:</p>
  
  <pre class="lang-py prettyprint-override"><code>wb2 = openpyxl.load_workbook('writeFormula.xlsx')
  sheet2 = wb2.active
  sheet2['A3'].value
  '=SUM(A1:A2)'  # python response
  </code></pre>
  
  <p>Any suggestions what am I doing wrong?</p>