title: get Excel sheet content on a pandas dataframe but with formulas not values
body: <p>I would like to have a pandas dataframe with the content of an excel file with
  the formulas if any.</p>
  <p>So from this <code>test.xlsx</code> file:</p>
  <pre><code>&gt;&gt;&gt; from openpyxl import load_workbook
  &gt;&gt;&gt; sheet = load_workbook('test.xlsx', data_only=False).active
  &gt;&gt;&gt; for i in range(1, 4, 1):
  &gt;&gt;&gt;     for j in range(1, 3, 1):
  &gt;&gt;&gt;         print(sheet.cell(i, j).value)
  &gt;&gt;&gt;         
  4
  a
  3
  b
  =A1+A2
  =B1&amp;&quot;_&quot;&amp;B2
  &gt;&gt;&gt;
  </code></pre>
  <p>How can I get this kind of pandas dataframe ?:</p>
  <pre><code>In [8]: df
  Out[8]:
            4              a
  0         3              b
  1  '=A1+A2'   '=B1&amp;&quot;_&quot;&amp;B2'
  </code></pre>
  <p>For now I only get values::</p>
  <pre><code>In [6]: import pandas as pd
  In [7]: df = pd.read_excel('test.xlsx')
  In [8]: df 
  Out[8]:
     4    a
  0  3    b
  1  7  a_b
  </code></pre>
  <p>Note: I'm on Linux so I can not use <code>xlwings</code></p>