docs: def flatten_fieldsets(fieldsets):
      """Returns a list of field names from an admin fieldsets structure."""
      field_names = []
      for _, opts in fieldsets or ():
          if 'fieldsets' in opts:
              field_names += flatten_fieldsets(opts.get('fieldsets'))
          else:
              for field in opts.get('fields', ()):
                  if isinstance(field, (list, tuple)):
                      field_names.extend(field)
                  else:
                      field_names.append(field)
      return field_names