title: hide label in django admin fieldset readonly field
body: <p>I have a really annoying issue that I can't hide a label in the admin if the field is readonly:</p>
  
  <pre><code>class Observable(Model):
      constraints=ManyToManyField('Constraint')
  
  class ObservableAdmin(MPTTModelAdmin):
  
      form=ObservableAdminForm
      fieldsets =[('other fields',{}),
                  ('All Constraints...:', {'fields':('constraints',)}),] 
      readonly_fields = ['constraints'] # I want to hide the "Constraints: " label
  
  class ObservableAdminForm(ModelForm):
  
      class Meta:
      model=Observable
      fields=('parent', 'name', 'alias', 'comments', 'constraints')
  
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
         # CAN'T DO self.fields['constraints'].label=''
  </code></pre>
  
  <p>The problem is that readonly field defined in the admin don't show up in the modelform fields at all. </p>
  
  <p>The django admin correctly displays the constraints as a comma separated block of text, but it shows the "Constraints" label which is redundant with "All Constraints..." in the fieldset. Can anyone suggest a workaround?</p>
  
  <p>thanks,
  Danny</p>