Source code that has `@final` decorator does not require `@final` in stub

[sobolevn]

Today I got hit by this. This was the source code:

from typing import final

@final
class A: ...

This was the stub:

class A: ...

But, stubtest allowed this.
I think that we should also check for __final__ attribute.

For example, some stdlib code already has @final decorators applied, like: https://github.com/python/cpython/blob/e1e9bab0061e8d4bd7b94ed455f3bb7bf8633ae7/Lib/asyncio/timeouts.py#L26

I will send a PR with the fix.