Enable Final instance attributes for attrs

[Tinche]

A quick patch to enable the following scenario:

@define
class C:
    a: Final[int]  # `a` is a final instance attribute

There are some edge cases I haven't covered here that would be complex to handle and not add much value IMO, so I think this will be useful like this.

For example:

@define
class C:
    a: Final[int] = field(init=False)

This is technically not sound, but there might be an __attrs_post_init__ on the class or one of its ancestors and that function might set it.

[JukkaL]

This seems to potentially modify the symbol node of a base class. That is not safe to do, since we may have serialized an earlier version of the class, etc. So probably best to do this only for attributes defined in the current class?

[Tinche]

@JukkaL Good catch, fix applied.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉