docs: def add_to_class(cls, name, value):
          """
          Django 1.10 doesn't allow us to modify `_base_manager` attr:
          https://docs.djangoproject.com/en/1.10/topics/db/managers/#django.db.models.Model._base_manager
          """
          if name == '_base_manager':
              if not value.name:
                  value.name = name
              value.model = cls
              setattr(cls._meta, 'base_manager_name', 'normal')
          else:
              super(VersionModelMeta, cls).add_to_class(name, value)