Use Enum for msg_types

[echarles]

The msg_types would deserve some Enum

ipykernel/ipykernel/kernelbase.py

Lines 197 to 207 in c9e2917

	msg_types = [
	'execute_request', 'complete_request',
	'inspect_request', 'history_request',
	'comm_info_request', 'kernel_info_request',
	'connect_request', 'shutdown_request',
	'is_complete_request', 'interrupt_request',
	# deprecated:
	'apply_request',
	]
	# add deprecated ipyparallel control messages
	control_msg_types = msg_types + ['clear_request', 'abort_request', 'debug_request']

This has been spotted by @Carreau while reviewing #805 in this comment #805 (comment)