title: isinstance(datetime.datetime, DateTimeField.value) always returns false for Django DateTimeField
body: <p>I think there is probably something very simple I'm missing here but I can't find it. </p>
  
  <p>I have a custom admin command in my django project that should iterate through instances of a <code>Record</code>'s model and write them to an xlsx file. </p>
  
  <p>I am using <code>xlsxwriter</code>, and as a result, I need to convert the <code>datetime.datetime</code> values in the <code>Record</code> instances to naive objects. So I want to say "When you encounter a <code>DateTimeField</code> in the <code>Record</code> model use <code>value.replace(tzinfo=None)</code> to make the object naive before writing it".</p>
  
  <p>For some reason, my <code>if isinstance(value, datetime.datetime)</code> is never returning true and I get a write error from <code>xlsxwriter</code> about the naive/aware <code>datetime</code> object.</p>
  
  <p>Here is the relevant code:</p>
  
  <p>sampleadmincommand.py</p>
  
  <pre><code>import pytz
  import datetime
  ....
  for i, row in enumerate(list(Record.objects.all())):
  
      for j,field in enumerate(list(Record._meta.get_fields())):
              value = getattr(row,field.name)
              if isinstance(value, datetime.datetime):
                  value.replace(tzinfo=None)
                  worksheet.write(i, j, value)
  
              else:
                  worksheet.write(i, j, value)
  </code></pre>
  
  <p>models.py</p>
  
  <pre><code>class Record(models.Model):
  
      time = models.DateTimeField()
      manHoursScheduled = models.FloatField()
      manHoursWorked = models.FloatField()
      carsOvernight = models.IntegerField()
      carsEvent = models.IntegerField()
      dailyTransientIncome = models.FloatField()
      dailyTransientCars = models.FloatField()
      dailyTransientIncomePerCar = models.FloatField()
  
      nightlyTransientIncome = models.FloatField(null=True)
      nightlyTransientIncomePerCar = models.FloatField(null=True)
  
      eventName = models.TextField(null=True)
  
      facility = models.ForeignKey(Facility)
  
      def __str__(self):
          return self.facility.name
  </code></pre>
  
  <p>I'm pretty stumped as to what I'm doing wrong here, so I would really appreciate any help!</p>