Set required=False on is_superuser field on user form.

accounts/viewsets.py

 class UserForm(forms.ModelForm):
     username = UsernameField()
     email = forms.EmailField(required=True)
-    is_superuser = forms.BooleanField(label='Superuser')
+    is_superuser = forms.BooleanField(label='Superuser', required=False)
     password1 = forms.CharField(
         label="Password",
         strip=False,