title: pandas: convert index type in multiindex dataframe
body: <p>Hi have a multiindex dataframe:</p>
  
  <pre><code>tuples = [('YTA_Q3', 1), ('YTA_Q3', 2), ('YTA_Q3', 3), ('YTA_Q3', 4), ('YTA_Q3', 99), ('YTA_Q3', 96)]
  # Index
  index = pd.MultiIndex.from_tuples(tuples, names=['Questions', 'Values'])
  # Columns
  columns = pd.MultiIndex.from_tuples([('YTA_Q3', '@')], names=['Questions', 'Values'])
  # Data
  data = [29.014949,5.0260590000000001,
    6.6269119999999999,
    1.3565260000000001,
    41.632221999999999,
    21.279499999999999]
  
  df1 = pd.DataFrame(data=data, index=index, columns=columns)
  </code></pre>
  
  <p>How do I convert the inner values of the df's index to str? </p>
  
  <p>My attempt:</p>
  
  <pre><code>df1.index.astype(str) 
  </code></pre>
  
  <p>returns a TypeError</p>