title: replace values in dataframe here rowname is equals to column name
body: <p>I have a dataframe with binary values. I want to change values to zero where row and column name is same. What is the easiest way to do this?</p>
  
  <pre><code>tmp_dict= {'x':[0,0,0,0,0],'y':[1,0,1,0,0],'z':[1,1,1,1,1],'p':[0,1,1,0,1],'q':[0,1,1,1,1]}
  mydf= pd.DataFrame(tmp_dict, columns=['x','y','z','p','q'], index=['x','y','z','p','q'])
  mydf
     x  y  z  p  q
  x  0  1  1  0  0
  y  0  0  1  1  1
  z  0  1  1  1  1
  p  0  0  1  0  1
  q  0  0  1  1  1
  </code></pre>
  
  <p>Desired dataframe:</p>
  
  <pre><code>   x  y  z  p  q
  x  0  1  1  0  0
  y  0  0  1  1  1
  z  0  1  0  1  1
  p  0  0  1  0  1
  q  0  0  1  1  0
  </code></pre>