Reject invalid override of a property with a declared attribute

Mypy doesn't complain about this, even though it fails at runtime with an exception:

class B:
    @property
    def x(self) -> int:
        return 0

class C(B):
    x: int
    def __init__(self) -> None:
        self.x = 4

C()

Exception:

Traceback (most recent call last):
  File "/Users/jukka/src/mypy/t/t5.py", line 11, in <module>
    C()
  File "/Users/jukka/src/mypy/t/t5.py", line 9, in __init__
    self.x = 4
    ^^^^^^
AttributeError: property 'x' of 'C' object has no setter

This doesn't generate an exception at runtime, however (and mypy correctly accepts this):

class B:
    @property
    def x(self) -> int:
        return 0

class C(B):
    x: int = 4

C()

Also, this already generates an error from mypy, as expected:

class B:
    @property
    def x(self) -> int:
        return 0

class C(B):
    def __init__(self) -> None:
        self.x = 4  # error: "x" is read-only

It seems that mypy should only allow overriding a read-only property with an attribute declared in the class body if the attribute has an initializer in the class body. For consistency, we should probably also use the same rule for overriding read-write properties.

Beginne damit, die drei Vererbungsbeispiele aus dem Issue zu reproduzieren, und untersuche mypy's Umgang mit deklarierten Attributen, die Properties überschreiben. Füge Abdeckung für den fehlschlagenden Fall der Instanzzuweisung und den akzeptierten Fall des Initialisierers im Klassenrumpf hinzu und überprüfe anschließend, dass schreibgeschützte und schreibbare Properties der vorgesehenen Regel folgen.