[stubtest] Support @final decorator on methods as well

[sobolevn]

After #14922 is merged, I think we can also add a check that this source code has correct stubs:

class A:
    @final
    def method(self): ...

Right now stubs do not have to add @final decorator, stubtest will allow both versions.

[sobolevn]

After working on for a short time, I've noticed several problems:

1. @property is not supported
2. @staticmethod is not supported
3. @classmethod is not supported

This happens, because __final__ cannot be set on property objects:

See:

>>> def my_final(f):
...    f.__final__ = True
...    return f

>>> class P:
...    @my_final
...    @property
...    def p(self) -> int: ...
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in P
  File "<stdin>", line 2, in my_final
AttributeError: 'property' object has no attribute '__final__'

But, @classmethod and @staticmethod actually change the object when using getattr, this is why this happens:

>>> class St:
...   @my_final
...   @staticmethod
...   def m() -> None: ...
... 
>>> St.m
<function St.m at 0x109eeb400>
>>> St.m.__final__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'function' object has no attribute '__final__'. Did you mean: '__init__'?

>>> St.__dict__['m']
<staticmethod(<function St.m at 0x109eeb400>)>
>>> St.__dict__['m'].__final__
True

This needs:

1. Special-casing properties. They cannot be supported right now
2. Magic getattr_static to get methods from the class

This is quite complicated for a Sunday's evening project, so - please feel free to continue it if someone finds it useful.

[AlexWaygood]

To update on the status here: following #14951, this is now fully implemented except for cases where the method is overloaded in the stub (implementing that is blocked by #14950).