title: Python - Openpyxl - "UserWarning: Unknown extension" issue
body: <p>I am trying to learn <code>Python</code> (day 2) and am hoping to practice with <code>Excel</code> books first as this is where I am comfortable/fluent. </p>
  
  <p>Right off the bat I am having an error that I don't quit understand when running the below code: </p>
  
  <pre><code>import openpyxl
  
  wb = openpyxl.load_workbook("/Users/Scott/Desktop/Workbook1.xlsx")
  
  print(wb.sheetnames)
  </code></pre>
  
  <p></p>
  
  <p>This does print my sheet names as requested, but it is followed by:</p>
  
  <pre><code>/Users/Scott/PycharmProjects/Excel/venv/lib/python3.7/site-packages/openpyxl/worksheet/_reader.py:293: UserWarning: Unknown extension is not supported and will be removed
    warn(msg)
  </code></pre>
  
  <hr>
  
  <p>I have found other questions that point to slicers/conditional formatting etc, but that does not apply here. This is a book I just made and only added 3 sheets before saving. It has no data, no formatting, and the extension is valid. I have no add-ons installed on my excel either.</p>
  
  <p><strong>Any idea why why I am getting this error? How do I resolve?</strong> </p>
  
  <p><a href="https://i.stack.imgur.com/ZLGkm.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ZLGkm.png" alt="enter image description here"></a></p>
  
  <p><em>Python: 3.7<br>
  openpyxl: 2.6</em></p>