title: Append a sheet to an existing excel file using openpyxl
body: <p>I saw this post to append a sheet using xlutils.copy:</p>
  
  <p><a href="https://stackoverflow.com/a/38086916/2910740">https://stackoverflow.com/a/38086916/2910740</a></p>
  
  <p>Is there any solution which uses only openpyxl? </p>