title: Overwrite maxlength/minlength of username by Django User model in the ModelForm
body: <p>Try to overwrite User models by the following code, but somehow I cannot overwrite the max_length and min_length of username.</p>
  
  <p>More specifically, when I check by <code>python manage.py shell</code>, I do overwrite them. But it seems has no effect on the html which was rendered(username maxlength is still 150).</p>
  
  <p>Don't know which parts get wrong, please help.</p>
  
  <pre><code>from django import forms
  from django.contrib.auth.models import User
  
  class RegistrationForm(forms.ModelForm):
  
  def __init__(self, *args, **kwargs):
      super(RegistrationForm, self).__init__(*args, **kwargs)
      email = self.fields['email']
      username = self.fields['username']
      email.required = True
      email.label_suffix = ' '
      username.label_suffix = ' '
      ######### this is not work!!!###############
      username.min_length = 6
      username.max_length = 30
      ############################################
  
  class Meta:
      model = User
      fields = ('username', 'email')
      labels = {
          'username': '帳號',
      }
      help_texts = {
          'username': '',
      }
  </code></pre>