Traverse the body of the node manually.

        If the first node is an expression which contains a string or bytes it
        marks that as a docstring.

def _visit_body(self, node):
        """
        Traverse the body of the node manually.

        If the first node is an expression which contains a string or bytes it
        marks that as a docstring.
        """
        if (node.body and isinstance(node.body[0], ast.Expr) and
                self.is_base_string(node.body[0].value)):
            node.body[0].value.is_docstring = True
            self.visit(node.body[0].value)

        for sub_node in node.body:
            self.visit(sub_node)