Start killing Python < 3.9 compatibility code

[DimitriPapadopoulos]

@denisri Should we get rid of this altogether?

# More supported types, in Python < 3.9 style
Tuple = tuple
Dict = dict
Set = set

I'm afraid it could break compatibility with existing code.

[denisri]

I don't think they are used except perhaps in a couple of tests, and possibly in axon/capsul auto conversion functions: Capsul3 has not been used much yet, and the older API was not using these types (we were using traits). I can adapt Axon if needed.