ShellMessageType: "comm_close" | "comm_info_reply" | "comm_info_request" | "comm_msg" | "comm_open" | "complete_reply" | "complete_request" | "execute_reply" | "execute_request" | "history_reply" | "history_request" | "inspect_reply" | "inspect_request" | "interrupt_reply" | "interrupt_request" | "is_complete_reply" | "is_complete_request" | "kernel_info_reply" | "kernel_info_request" | "shutdown_reply" | "shutdown_request"

Shell message types.