title: Delete rows based on unique number of elements in a column pandas
body: <p>Given a data frame with two columns <code>A</code> and <code>B</code>:</p>
  
  <pre><code>df = 
  
  A      B
  cat    3
  cat    4
  cat    2
  bird   1
  bird   3
  bird   2
  bird   5
  bird   3
  </code></pre>
  
  <p>I want to delete rows if the number of unique elements in column <code>A</code> is less <code>3</code>
  len of <code>cat</code> is - 3 (delete)
  len of <code>bird</code> is - 5 (keep)</p>
  
  <p>desired output:</p>
  
  <pre><code>df = 
  
  A      B
  bird   1
  bird   3
  bird   2
  bird   5
  bird   3
  </code></pre>