[stubtest] Respect @final runtime decorator, enforce it in stubs

[sobolevn]

Closes #14921

[AlexWaygood]

Nice idea! Should we do the same for @typing(_extensions).override?

[AlexWaygood]

Also:

• Methods can also be decorated with @final/@override, maybe we should also verify those in the same way?
• We should add a comment next to this line, saying that we give a specialised error for classes decorated with @final elsewhere:
  

  mypy/mypy/stubtest.py

  Line 1342 in 59886d2

  "__final__",

[sobolevn]

@AlexWaygood I don't see much value in @override decorator support, but I guess it is kinda easy to add. So, why not?

I will also work on @final decorator support on methods in the next PR. Nice idea!

Thanks a lot for the review, I am going to merge this when the CI is green 👍