title: Convert all columns from int64 to int32
body: <p>We all now the question: <a href="https://stackoverflow.com/questions/15891038/change-data-type-of-columns-in-pandas">Change data type of columns in Pandas</a> where it is really nice explained how to change the data type of a column, but what if I have a dataframe <code>df</code> with the following <code>df.dtypes</code>:</p>
  
  <pre><code>A  object
  B   int64
  C   int32
  D   object
  E   int64
  F  float32
  
  </code></pre>
  
  <p>How could I change this <strong>without explicity</strong> mention the column names that all <code>int64</code> types are converted to <code>int32</code> types?
  So the desired outcome is:</p>
  
  <pre><code>A  object
  B   int32
  C   int32
  D   object
  E   int32
  F  float32
  </code></pre>