title: Csv DictReader throwing error for fieldnames
body: <p>I have a piece of code that has been working for a while that uses Python’s DictReader.</p>
  
  <p>The code initializes the csv reader, <code>csv_reader = csv.DictReader(my_csv)</code> and then I access <code>csv_reader.fieldnames</code>. Historically this has been working fine. </p>
  
  <p>However today it started throwing this error <code>iterator should return strings, not bytes (did you open the file in text mode?)</code> when I try to access <code>csv_reader.fieldnames</code>.</p>
  
  <p><code>csv_reader.__dict__</code> shows an object with an attribute <code>_fieldnames</code>, and it is empty. I’m not sure why this changed or what I can do to resolve it, any suggestions are welcome.</p>