title: Pandas group by on one column with max date on another column python
body: <p>i have a dataframe with following data :</p>
  
  <pre><code>invoice_no  dealer  billing_change_previous_month        date
         110       1                              0  2016-12-31
         100       1                         -41981  2017-01-30
        5505       2                              0  2017-01-30
        5635       2                          58730  2016-12-31
  </code></pre>
  
  <p>i want to have only one dealer with the maximum date . The desired output should be like this :</p>
  
  <pre><code>invoice_no  dealer  billing_change_previous_month        date
         100       1                         -41981  2017-01-30
        5505       2                              0  2017-01-30
  </code></pre>
  
  <p>each dealer should be distinct with maximum date,
  thanks in advance for your help.</p>