[mypyc] Fix property assignment when the setter accepts a wider type than the getter

[VaggelisD]

When a property setter accepts a wider type than the getter returns, mypyc coerced the assigned value to the getter's type, so valid assignments failed at runtime:

from typing import Union

class C:
    def __init__(self) -> None:
        self._x = 0

    @property
    def x(self) -> int:
        return self._x

    @x.setter
    def x(self, value: Union[int, str]) -> None:
        self._x = int(value)

def set_x(c: C) -> int:
    c.x = "1"  # TypeError: int object expected; got str
    return c.x

This uses the setter's argument type instead, both when coercing the assigned value and in the generated CPY_SET_ATTR call.

[JukkaL]

Thanks for the fix, looks good!