title: xlwt write excel sheet on the fly
body: <p>I am used to creating a spreadsheet in the following way:</p>
  
  <pre><code>    wbk = xlwt.Workbook()
      earnings_tab = wbk.add_sheet('EARNINGS')
      wbk.save(filepath)
  </code></pre>
  
  <p>Is there any way to not save to file to a filepath, and instead write it on-the-fly to a user who downloads the file? Or do I need to save it as a tmp file and then serve that to the user?</p>