title: How to convert a matrix into column array with PANDAS / Python
body: <p>I need to convert a matriz (below) into a 3 column list. </p>
  
  <p>I currently have a DataFrame as this:</p>
  
  <pre><code>     A   B   C  ...
  a    1   3   4  ...
  b    4   0   6  ...
  c    9   8   0  ...
  </code></pre>
  
  <p>and I need to get a results as this: </p>
  
  <pre><code>Source   Target    Weight
    a        A          1
    a        B          3
    a        C          4
    b        A          4
    b        C          6
    c        A          9
    c        B          8
  </code></pre>
  
  <p>I've been trying to use pandas.pivot_table function, but I can't get the result needed. </p>