[mypyc] Optimize calls to final classes

[jairov4]

Fixes #9612

This change allows to gain more efficiency where classes are annotated with @final bypassing entirely the vtable for method calls and property accessors.

For example:
In

@final
class Vector:
    __slots__ = ("_x", "_y")
    def __init__(self, x: i32, y: i32) -> None:
        self._x = x
        self._y = y

    @property
    def y(self) -> i32:
        return self._y

def test_vector() -> None:
    v3 = Vector(1, 2)
    assert v3.y == 2

The call will produce:

...
cpy_r_r6 = CPyDef_Vector___y(cpy_r_r0);
...

Instead of:

...
cpy_r_r1 = CPY_GET_ATTR(cpy_r_r0, CPyType_Vector, 2, farm_rush___engine___vectors2___VectorObject, int32_t); /* y */
...

(which uses vtable)

[jairov4]

hello @JukkaL
can you review this PR?

[JukkaL]

Thanks for the PR! Looks good, just left a few minor comments.

[jairov4]

@JukkaL Hi, I have addressed your comments

[JukkaL]

Thanks!