title: Convert string with NaNs to int in pandas
body: <p>I have a pandas dataframe, all the values are strings. Some are 'None's, and the rest are integers but in string format, such as '123456'. How can I convert all 'None's to np.nan, and others to integers, like, 123456.</p>
  
  <pre><code>df = {'col1': ['1', 'None'], 'col2': ['None', '123']}
  </code></pre>
  
  <p>Convert df to:</p>
  
  <pre><code>df = {'col1': [1, NaN], 'col2': [NaN, 123]}
  </code></pre>