title: Drop rows if not equal to another row pandas
body: <p>Here is a df :</p>
  
  <pre><code>col1    col2    col3
  A   A   23
  B   A   32
  C   D   43
  E   E   54
  </code></pre>
  
  <p>and the idea is to only keep rows where col1 and col2 are equal and get :</p>
  
  <pre><code>col1    col2    col3
  A   A   23
  E   E   54
  </code></pre>
  
  <p>I tried: </p>
  
  <pre><code>for col1,col2 in zip(df1.col1, df1.col2): 
      if col1 != col2:
          df2.drop(col1)
  </code></pre>