title: Half Hours choice fields for TimeField in Django model
body: <p>I want to have field choice in form that user can pick hours with half hours choice.
  I have this model and this <code>TimeField</code>: </p>
  
  <pre><code>class Sample:
     start_time = models.TimeField(choices=global_vars.TIME_MAP_HALF_HOURS_CHOICES,)
  </code></pre>
  
  <p>I want to show half hours time to customer. This is my Choices: </p>
  
  <pre><code>TIME_MAP_HALF_HOURS_CHOICES = (
      (datetime.time(00, 00, 00), '00:00'),
      (datetime.time(00, 30, 00), '00:30'),
      (datetime.time(1, 00, 00), '01:00'),
      (datetime.time(1, 30, 00), '01:30'),
      (datetime.time(2, 00, 00), '02:00'),
      (datetime.time(2, 30, 00), '02:30'),
      (datetime.time(3, 00, 00), '03:00'),
      (datetime.time(3, 30, 00), '03:30'),
      (datetime.time(4, 00, 00), '04:00'),
      (datetime.time(4, 30, 00), '04:30'),
      (datetime.time(5, 00, 00), '05:00'),
      (datetime.time(5, 30, 00), '05:30'),
      (datetime.time(6, 00, 00), '06:00'),
      (datetime.time(6, 30, 00), '06:30'),
      (datetime.time(7, 00, 00), '07:00'),
      (datetime.time(7, 30, 00), '07:30'),
      (datetime.time(8, 00, 00), '08:00'),
      (datetime.time(8, 30, 00), '08:30'),
      (datetime.time(9, 00, 00), '09:00'),
      (datetime.time(9, 30, 00), '09:30'),
      (datetime.time(10, 00, 00), '10:00'),
      (datetime.time(10, 30, 00), '10:30'),
      (datetime.time(11, 00, 00), '11:00'),
      (datetime.time(11, 30, 00), '11:30'),
      (datetime.time(12, 00, 00), '12:00'),
  )
  </code></pre>
  
  <p>But When I want to save object, the form not validate and raise error: <code>Choose a valid option.</code>
  How can I fix this error‌?</p>