title: 'Workbook' object has no attribute 'get_worksheet_by_name'
body: <pre><code>import xlsxwriter as xl
  wb = xl.Workbook(f, {'in_memory': True})
  sheet = wb.add_worksheet('example')
  sheet = wb.get_worksheet_by_name('example')
  </code></pre>
  
  <p>According to the <a href="http://xlsxwriter.readthedocs.io/workbook.html#workbook-get-worksheet-by-name" rel="nofollow">docs</a> the function is correct but I get an error. </p>