title: Django ChoicesField divider?
body: <p>I'm using a <code>ChoicesField</code> in my form, but I want to put a divider in it like this:</p>
  
  <pre><code>COUNTRIES = (
      ('CA', _('Canada')),
      ('US', _('United States')),
      (None, _('---')), # &lt;----------
      ('AF', _('Afghanistan')),
      ('AX', _('Aland Islands')),
      ('AL', _('Albania')),
      ('DZ', _('Algeria')),
      ('AS', _('American Samoa')),
      # ...
  
  class AddressForm(forms.Form):
      country = forms.ChoiceField(choices=COUNTRIES, initial='CA')
  </code></pre>
  
  <p>What's the easiest way to make that unselectable, or at least give an error if the user picks it?</p>