[mypyc] Use shared value for Final attribute initialized in class body

[JukkaL]

Use a module-level static for Final attributes initialized in class body:

class C:
    X: Final = [1]  # Only one list allocated

Previously such final attributes where stored in the per-instance struct, and a new object was allocated for each instance, which deviated from Python semantics and was inefficient. Now the initializer is only run once.

Final attributes that have a type with an overlapping error value like i64 are still not supported properly. This is left as follow-up work.

I used coding agent assist but reviewed the result manually and iterated on it.