title: Delete duplicate records in a pandas dataframe, but keep one based on alphabetical order
body: <p>I cross-joined a query and returned it as a DataFrame in Python. Now I have a dataframe df that looks like this:</p>
  
  <pre><code>df = pd.DataFrame([['Peter', 'Tom',1], ['Sam', 'Ed',2], ['Tom', 'Peter',1], ['Ed', 'Sam',2]], columns=["Person 1", "Person 2", "Value"])
  
  
  df 
     Person1 Person2 Value
  0   Peter   Tom    1
  1   Sam     Ed     2
  2   Tom     Peter  1
  3   Ed      Sam    2
  </code></pre>
  
  <p>However, I need to remove the duplicates and keep only record for each pair. I want to keep the pair based on whose name is first in the alphabet, and have my final dataframe look like this:</p>
  
  <pre><code>df_final
  
     Person1  Person2 Value
  0   Ed      Sam      2
  1   Peter   Tom      1
  </code></pre>