title: Django admin form validation
body: <p>In my django app, I want to add a validation to the automatically generated admin form of a model. I want to raise a <code>ValidationError</code> if the attribute <code>title</code> equals "test". I have tried the following in <code>admin.py</code>, but nothing happens if the title is "test". Could anyone help, please?</p>
  
  <pre><code>from django.contrib import admin
  from django.forms import ModelForm, ValidationError
  
  from .models import MyModel
  
  class MyModelAdminForm(ModelForm):
      class Meta:
          model = MyModel
          fields = '__all__'
  
      def clean(self):
          cleaned_data = super().clean()
          title = cleaned_data.get('title')
          if title == 'test':
              raise forms.ValidationError('invalid!')
          return cleaned_data
  
  class MyModelAdmin(admin.ModelAdmin):
      form = MyModelAdminForm
  
  admin.site.register(MyModel)
  </code></pre>