title: Openpyxl 2.4.1 can't get rid of green arrows in Excel after setting new number_format
body: <p>I am trying to get rid of green arrows in Excel standing for:
  'The number in this cell is formatted as text or preceded by an apostrophe.'</p>
  
  <p>Working with Openpyxl 2.4.1 and Python 3.5.2.</p>
  
  <p>For now function looks like this (however I have tried many different versions):</p>
  
  <pre><code>def price():
      cena = input("Cena: ")
      cena = cena.replace('.', ',')
      cell = 'D' + str(row)
      arkusz1.active[cell].number_format = '0.00'
      arkusz1.active[cell] = cena
      print("Wprowadzono Cenę. \n")
  </code></pre>
  
  <p>After this there is indeed cell with number format in excel
  <a href="https://i.stack.imgur.com/HJaT1.jpg" rel="nofollow noreferrer">enter image description here</a></p>
  
  <p>but the green arrow is still present and the value's format isn't a number - can't be used in formulae.</p>
  
  <p>Any solution?</p>