Final variable is mutable when using structural subtyping #10850

Bug Report

Under structural subtyping with a variable narrowed to Final, it's possible to modify the var contents

To Reproduce

# poc.py
from dataclasses import dataclass
from typing import Protocol, Final

@dataclass
class FinalY:
    y: Final[int] = 0

class Y(Protocol):
    y: int

def f(x: Y) -> Y:
    x.y = 42
    return x

print(f(FinalY(17)))

$ mypy poc.py
Success: no issues found in 1 source file

$ python poc.py
FinalY(y=42)

Expected Behavior

Class with a variable Final[T] should not be a structural subtype of a protocol with variable T

Your Environment

• Mypy version used: 0.910
• Python version used: 3.9.6

This bug was inspired by @erictraut's example of how to deal with other kind of issue