Check whether an ``{% extends variable %}`` is used in the template.

    :type extend_node: ExtendsNode

def _is_variable_extends(extend_node):
    """
    Check whether an ``{% extends variable %}`` is used in the template.

    :type extend_node: ExtendsNode
    """
    if django.VERSION < (1, 4):
        return extend_node.parent_name_expr  # Django 1.3
    else:
        # The FilterExpression.var can be either a string, or Variable object.
        return not isinstance(extend_node.parent_name.var, six.string_types)