Add type hints to Subscription

[okapies]

• Add type hints to Subscription
• Add SubscriptionHandler protocol class for type hinting

[oroulet]

to solve that error. use if typing.TYPE_CHECKING: import whatever for typing

[okapies]

@oroulet Some tests failed because they pass Node to subscribe_events directly instead of NodeId:

opcua-asyncio/tests/test_subscriptions.py

Lines 733 to 740 in 39e0710

	etype = await opc.server.create_custom_event_type(2, 'MyEvent', ua.ObjectIds.BaseEventType, [
	('PropertyNum', ua.VariantType.Float),
	('PropertyString', ua.VariantType.String)
	])
	evgen = await opc.server.get_event_generator(etype)
	myhandler = MySubHandler()
	sub = await opc.opc.create_subscription(100, myhandler)
	handle = await sub.subscribe_events(evtypes=etype)

against the pydoc:

    async def subscribe_events(
        ...
    ) -> int:
        """
        ...
        :param evtypes: ua.ObjectIds or ua.NodeId

Do you think we still need to support Node, or fix the tests to pass NodeId?

[oroulet]

We have been supporting both for ages, so we should continue with it

[okapies]

Ok, I'll fix both the implementation and the type hint.

[okapies]

Thank you!