Allow setting a final instance attribute in `__set_name__`?

Currently, Mypy reports the following problems:

from typing import Any, Final, Type

class D:
    name: Final[str]  # error: Final name must be initialized with a value  [misc]

    def __set_name__(self, owner: Type[Any], name: str) -> None:
        self.name = name  # error: Cannot assign to final attribute "name"  [misc]

I know PEP 591 does not mention __set_name__, but I think it could be helpful if Mypy would handle it like __init__ regarding the one-time initialisation of instance attributes.

Start by reproducing the Python example from the issue and compare it with Mypy’s existing handling of Final instance attributes in init. Investigate where Final assignment rules and set_name behavior are analyzed. Done means the proposed one-time initialization behavior is implemented with regression coverage and the example no longer reports either error.