title: openpyxl giving None value when calling value of formula cell
body: <p>When I'm trying to access the value of a cell that has a formula (<code>=E3*X</code>) I get <code>None</code> even with <code>data_only=True</code></p>
  <pre><code>from openpyxl import load_workbook
  
  book = load_workbook(&quot;sales.xlsx&quot;, data_only=True)
  sheet = book['sales']
  print(sheet[&quot;G3&quot;].value)
  book.close()
  
  &gt;&gt; Output: None
  &gt;&gt; Cell's actual value: =E3*7.5 (60, E3=8)
  </code></pre>