title: Reshape Pandas dataframe that contains all years in one column and months separated across multiple columns
body: <p>I have a dataframe that contains sales per product for different time periods over different years:</p>
  <pre><code>df = pd.DataFrame({'Product': {0: '1', 1: '1', 2: '2', 3: '2'},
                     'Year': {0: 2000, 1: 2001, 2: 2001, 3:2002},
                     'Jan-Feb': {0: 2, 1: 4, 2: 2, 3:4},
                      'Mar-Apr': {0: 1, 1: 2, 2: 1, 3:6}})
  </code></pre>
  <p><code>df</code> has products the sales of products 1 and 2 for each <code>year</code> but the periods of time within a year are presented in different columns. In this example I present two columns but there are four more representing the remaining periods of two consecutive months within a year.</p>
  <p>I would like to reshape <code>df</code> to look like this:</p>
  <pre><code>df2 = pd.DataFrame({'Product': {0: '1', 1: '1', 2: '1', 3: '1', 4: '2', 5: '2', 6: '2', 7: '2'},
                     'Year': {0: 2000, 1: 2000, 2: 2001, 3:2001,4: 2001, 5: 2001, 6: 2002, 7:2002},
                     'Period': {0: 'Jan-Feb', 1: 'Mar-Apr', 2: 'Jan-Feb', 3:'Mar-Apr', 4: 'Jan-Feb', 5: 'Mar-Apr', 6: 'Jan-Feb', 7:'Mar-Apr'},
                     'Sales': {0: 2, 1: 1, 2: 4, 3: 2, 4: 2, 5: 1, 6: 4, 7: 6}})
  </code></pre>
  <p>In this case, month periods are all in one column and sales in a different column. I have tried different forms of reshaping but I'm clearly missing something as I haven't been successful in finding one that does what I want.</p>