title: Django modelform remove "required" attribute based on other field choice
body: <p>I have ModelForm with several fields. Some of fields are required, some not. Also I have Select field with different choices, and I want to make some of fields "required" or not based on this Select field choice.</p>
  
  <p>I tried in clean() method of Form</p>
  
  <pre><code>def clean(self):
      cleaned_data = self.cleaned_data
      some_field = cleaned_data.get("some_field")
      if some_field == 'some_value':
            self.fields['other_field'].required = False
      return cleaned_data
  </code></pre>
  
  <p>but it doesn't work</p>