fix: Don't turn items annotated as InitVar into dataclass members

[has2k1]

This PR addresses the case of dataclass init-only variables also being treated as class members. i.e

from dataclasses import dataclass, InitVar

@dataclass
class Point:
    x: float
    y: float
    z: InitVar[float]

is equivalent (compatible) to

class Point:
    def __init__(self, x: float, y: float, z: float):
       self.x = x
       self.y = y

The class is not expected to have member z (instance.z).

[pawamoy]

I really know nothing about dataclasses haha! Thanks 🙂

[has2k1]

I really know nothing about dataclasses haha! Thanks 🙂

I have just accepted them as a Whac-A-Mole situation.

[pawamoy]

OK, LGTM, lets squash and merge 🙂

[pawamoy]

Ah wait, there are some linting warnings, let me fix those.