title: Check if DataFrame column contains only strings
body: <p>Let's say I have a DataFrame <code>df</code>:</p>
  
  <pre><code>     A     B
  0    a     1
  1    a1    3
  2    b_    5
  3    c(    7
  </code></pre>
  
  <p>I would like to get only these elements (e.x as a list) from the the column <code>A</code> that are strings, so in this case only <code>a</code> would be returned. I thought of <code>.str.contains()</code> but I'd rather use something that bases on types.</p>