title: Count number of categories and listwise delete if condition not satisfied
body: <p>Given: </p>
  
  <pre><code>import pandas as pd
  
  lis1= ('apple','orange','strawberry','strawberry','strawberry','apple','orange','orange','orange','strawberry')
  lis2= ("lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review")
  
  pd.DataFrame({'category':lis1, 'review': lis2})
  
       category              review
  0       apple  lorem ipsum review
  1      orange  lorem ipsum review
  2  strawberry  lorem ipsum review
  3  strawberry  lorem ipsum review
  4  strawberry  lorem ipsum review
  5       apple  lorem ipsum review
  6      orange  lorem ipsum review
  7      orange  lorem ipsum review
  8      orange  lorem ipsum review
  9  strawberry  lorem ipsum review
  </code></pre>
  
  <p>Need: </p>
  
  <pre><code>lis1= ('orange','strawberry','strawberry','strawberry','orange','orange','orange','strawberry')
  lis2= ("lorem ipsum review","lorem ipsum review", "lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review","lorem ipsum review")
  
  pd.DataFrame({'category':lis1, 'review': lis2})
  
       category              review
  0      orange  lorem ipsum review
  1  strawberry  lorem ipsum review
  2  strawberry  lorem ipsum review
  3  strawberry  lorem ipsum review
  4      orange  lorem ipsum review
  5      orange  lorem ipsum review
  6      orange  lorem ipsum review
  7  strawberry  lorem ipsum review
  </code></pre>
  
  <p>I need the code to do a count of the unique categories (nunique()) and delete the categories that only appear less than 3 times. The example shows that since apple is the only category that appears twice, a listwise delete was applied.</p>